Codechange: Remove direct access to _colour_gradient.

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

Codechange: Make all NWidgetPart arrays constexpr.

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

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 __FILE__/__LINE__ of WindowDesc to WindowDesc.

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

Add: data parameter in Window::Close method.

This allows passing data when closing a window, e.g. to indicate how it was closed.

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.

Change: Add window description flag to disallow interactive window closing. (#11008)

This replaces/simplifies testing for a closebox to allow closing a window with right-click, and testing for specific window classes when closing all windows by hotkey.

This allows right-click closing of dropdowns and the high-score window.

Change: Standardize progress bar layout.

Progress bars are drawn differently depending on when it was added, with
different layouts and sizes.

This change adds a standard padding size to use, and makes all progress
bars visually similar, with scaled padding.

Change: Use Rect helpers for widget drawing.

This replaces repetitive and sometimes unwieldy use of constants.

Change: Rename some freetype things to fontcache.

The font cache supports more than just FreeType as a font provider, but still used freetype in some naming.

This now uses more suitable terms.

Fix: if bootstrap failed, it could end with an empty screen instead of error

There are various of ways bootstrap can fail:
- Failing network connection
- Incomplete download
- No write permissions
- Disk full
- (others I forgot)

They all result in a screen with no windows. To ensure we at least
always show something when anything bad happens, if the bootstrap
is not successful, show a screen what the next step for the human
should be.

Fix a49fdb7ebb: bootstrap crash when trying to load new baseset (#8353)

Using nullptr as "name" crashes on "name.empty()". Use an empty
string instead.

Remove: ENABLE_NETWORK switch

This switch has been a pain for years. Often disabling broke
compilation, as no developer compiles OpenTTD without, neither do
any of our official binaries.

Additionaly, it has grown so hugely in our codebase, that it
clearly shows that the current solution was a poor one. 350+
instances of "#ifdef ENABLE_NETWORK" were in the code, of which
only ~30 in the networking code itself. The rest were all around
the code to do the right thing, from GUI to NewGRF.

A more proper solution would be to stub all the functions, and
make sure the rest of the code can simply assume network is
available. This was also partially done, and most variables were
correct if networking was disabled. Despite that, often the #ifdefs
were still used.

With the recent removal of DOS, there is also no platform anymore
which we support where networking isn't working out-of-the-box.

All in all, it is time to remove the ENABLE_NETWORK switch. No
replacement is planned, but if you feel we really need this option,
we welcome any Pull Request which implements this in a way that
doesn't crawl through the code like this diff shows we used to.

(svn r23525) -Codechange: make Window::flags4 WindowFlags instead of uint16, with only values known in WindowFlags (and move out 2 timers to their own variable)
-Codechange: rename Window::flags4 to Window::flags
-Codechange: move some non-inline functions from .hpp to .cpp