Codechange: Remove direct access to _colour_gradient.

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

Codechange: Add method to replace the content of a dropdown menu.

If necessary the dropdown list window will be resized and scrollbar enabled/disabled.

Codechange: Add parameter to persist dropdown menus.

If a dropdown menu is set to persist, it will not close when an item is selected. It will close as normal if the window loses focus.

Closing the list is the responsibility of the caller.

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: Add DropDownIcon constructor to override dimension.

This avoids the need to construct a DropDownIcon and set the dimension after.

Change: Use CRTP-mixins to compose dropdown list items.

This allows list items to built from component parts as required, and additional
functionality is added:

* Icons and text can be positioned at the start or end of the space (templated.)
* Font size of text can be changed (templated.)
* Palette of sprites can be set (runtime.)

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(...)`.

Fix d42a78f: Some raw drop down list strings may need token processing. (#11400)

Storing the raw string without processing though GetString() caused token
processing to be skipped.

Codechange: Remove unused parameter for Height()

DropDownListItem::Height does not need to take an argument so remove it

Signed-off-by: Bernhard Reutner-Fischer <rep.dot.nop@gmail.com>

Codechange: Make DropDownListStringItem preformat and remove other implementations. (#11063)

Having to choose between DropDownListStringItem, DropDownListCharStringItem, and DropDownListParamStringItem depending on whether to draw a StringID, a raw string, or a StringID with extra parameters was needlessly complex.

Instead, allow passing a StringID or raw string to DropDownListStringItem. This will preformat the StringID into a raw string, and can therefore accept parameters via the normal SetDParam mechanism.

This also means that strings no longer need to be formatted on every draw.

Change: Make all dropdown lists extend width if necessary.

This removes the auto_width parameter from ShowDropDown(At).

Fix: Incorrect vertical alignment of icon and text in DropDownListIconItem. (#9133)

This happens if the bounding dimensions are changed so that each item is the same size, as happens on the railtype/roadtype dropdown lists, as the vertical offset was calculated before this dimension is changed.

Codechange: remove special strings for language and resolutions (#8824)

As OpenTTD grew, we found other ways to do this, and we are no
longer in need for a hack like this.

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.

(svn r14004) -Codechange: Clean of drop down lists.
Move empty item drawing to base ListItem Draw() function.
Remove String() from base class.
Pass correct width to Draw().