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.

Fix #11577: Extra viewport opened in wrong location. (#11578)

ExtraViewportWindow calls IninitializeViewport() with focus as 0, which is ambiguous as focus should be either a TileIndex or a VehicleID.

Instead, pass the tile and let InitializeViewport() handle setting all the coordinates.

Codechange: Add __FILE__/__LINE__ of WindowDesc to WindowDesc.

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

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: introduce GetMainWindow() to properly account for nullptr checks

Some nullptr checks have been removed as they were not triggered with nullptr
with the null video driver and in dedicated server mode.

Fix #9995: Adjust viewport zoom level for HiDPI displays

On HiDPI screens the zoom level is increased for detailed rendering. This causes hard-coded zoom levels to be off by this adjustment. To fix these default zoom levels, we scale the zoom level based on `_gui_zoom` to get the scaled zoom level.

Codechange: Spell 'Viewport' consistently

Some places in the codebase misspell 'Viewport' as 'ViewPort' or 'view_port'.
This patch makes everything consistent.