Copy local theme overrides from Control to Window

[YuriSizov]

Closes #69308. This is probably the last missing feature from Windows related to theming.

While this PR manipulates a lot of lines, it doesn't do anything particularly smart. Most of the code is just copied and adapted from control.h/cpp. I've renamed some internal members in Control as well, for consistency. And moved some things in Window for better grouping. Documentation should also be in sync now.

godot.windows.editor.dev.x86_64_2022-11-29_22-57-09.mp4

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PS. My plan is to reduce this duplicated code significantly in future, when I can work on the first-class theme properties. But for now this will do.

[KoBeWi]

Changing overrides does not update the window immediately.

[YuriSizov]

Changing overrides does not update the window immediately.

Hmm, it clearly works with popups. Can it be an issue with the editor preview viewport?

[KoBeWi]

Seems like the window does not redraw on theme change. Changing size will refresh the style.

[YuriSizov]

There doesn't seem to be a dedicated method to update a window like there is for CanvasItems (queue_redraw), but we can consider that theme changes can cause window size changes and thus request that kind of update. I've hooked into an existing method that already does that for when new child nodes are added, because this call needs to be deferred otherwise we face a lot of lag.

godot.windows.editor.dev.x86_64_2022-12-07_20-49-32.mp4

[akien-mga]

Thanks!