Codechange: Remove direct access to _colour_gradient.

Access is now through GetColourGradient, which ensures parameters are in range.

Fix #12012: Crash when opening orders of another company. (#12013)

Orders window changes tooltip depending on vehicle type but did check if widget exists.

Codechange: Make all NWidgetPart arrays constexpr.

This ensures that the arrays are not created at runtime and prevents using non-constexpr values.

Codechange: Redefine some cargo-related values in terms of CargoID instead of CargoType.

Values used as special filter types are now defined as offsets from NUM_CARGO instead of confusingly reusing CARGO_NO_REFIT/CARGO_AUTO_REFIT types.

Codechange: Replace mishmash of types for widget index with WidgetID.

Indices were stored as int, but often passed around as uint/uint8_t and casts.

Now they should all use WidgetID.

Codechange: Remove FONT_HEIGHT_... macros. (#11481)

These make it look like we're dealing with a constant, but actually each is a call to `GetCharacterHeight(...)`.

Codechange: Add __FILE__/__LINE__ of WindowDesc to WindowDesc.

This is to allow unit-tests to produce more useful output.

Codechange: Avoid emplace_back(new()) into a unique_ptr. (#11384)

This could theoretically leave an unmanaged pointer in certain circumstances, and directly using
make_unique shows intent.

Codechange: Use begin/end of nwidget parts of begin/length.

This simplifies processing nwidget parts as, unlike the remaining length, the pointer to the end of the list never changes. This is the same principle as we use(d) for tracking end instead of length for C-style strings.

And this removes 160~ instances of the lengthof() macro.

Codechange: Reorganise hotkey initialisation. (#10951)

Hotkeys are now initialized inline, and use std::vector instead of
separate static C-arrays and std::string instead of char *. The list end
marker is no longer required.

Codechange: Use window parent association for dropdowns.

This replaces the separate window class and number properties, and
allows the window system to close dropdowns automatically.

Codechange: Use GetScrolled(Row/Item)FromWidget in more places.

In many instances the clicked row position is 'manually' calculated
instead of using the GetScrolledRowFromWidget helper function, with
variations on checks. Replace with the two helpers where possible.

Feature: Separate rail/road and sea/air velocity units, and add knots. (#10594)

This is achieved by packing vehicle type along with the velocity so that
the string system can decode and pick the appropriate unit.

Codechange: add non-nullptr asserts in cases where it should never be nullptr

Though where similar calls are checked for nullptr as in those instances of
the use of that function it can actually return nullptr. In other words, write
down the assumption that the function never returns nullptr in an assert.

Change: Use Rect helpers for widget drawing.

This replaces repetitive and sometimes unwieldy use of constants.

Codechange: Template DoCommandP to automagically reflect the parameters of the command proc.

When finished, this will allow each command handler to take individually
different parameters, obliviating the need for bit-packing.

Codechange: remove unneeded comparison and casts

Division by resize_y is already yielding an unsigned number, so when clicking in the WD_FRAMERECT_TOP you would already get a huge value, so sel would never be negative. So, leave sel an unsigned number and remove the <= check.

Fix #7972: show invalid orders to stations that don't accept your vehicle (#8516)

Before it was shown as a normal order, but the vehicle was skipping
it. This was rather unclear to the user. Now it is red and contains
text with some hints what is going on.

The text is prefixed rather than post-fixed, as we have many
post-fixes already.

Fix fbbbc6e1931: Crash when creating orders to stations with relocated station sign. (#7755)

Station::xy is the position of the station sign, and not necessarily a tile of the station.

Fix #7692: Added industry tile to GetOrderCmdFromTile() (#7709)

Sending order command to an industry tile now checks if a neutral_station is available and sends the order to that station

Change: Always report error when ordering a road vehicle to wrong type of road stop. (#7316)

This was inconsistent before, depending on road/tram and articulated/single vehicle status.

Codechange: Switch DropDownList to directly use std::vector, thus making AutoDeleteSmallVector obsolete.

DropDownListItem are strongly managed using std::unique_ptr to ensure leak-free handling. Appropriate use
of move-semantics make intent a lot clearer than parameter comments and allows the compiler to generate
copy-free code for most situations.