Cleanup: Rename ExtensionRNA variables from ext to rna_ext

Makes it more clear that code using this is related to the RNA
integration of a type.
Part of T74432.

Also ran clang-format on affected files.

Cleanup: Rename bScreen variables from sc/scr to screen

Part of T74432.

Mostly a careful batch rename but had to do few smaller fixes.

Also ran clang-format on affected files.

Cleanup: Rename ScrArea variables from sa to area

Follow up of b2ee1770d4c3 and 10c2254d412d, part of T74432.
Now the area and region naming conventions should be less confusing.

Mostly a careful batch rename but had to do few smaller fixes.

Also ran clang-format on affected files.

Cleanup: Continue renaming ARegion variables from ar to region

Continuation of b2ee1770d4c3, now non-single word variables are also
renamed.
Part of T74432.

Also ran clang-format on affected files.

Fix accidentally reverted changes in VR merge due to merge error

dc2df8307f41 unintentionally reverted part of 07bdbeda8462.

Fix warnings caused by own earlier commit

Caused by 34465a7fb091.

VR: Refactor DirectX context management

All DirectX management happens on Ghost level now, higher level code can
just assume everything is OpenGL (except of the upside-down drawing that
still needs to be done for DirectX). This is similar to how the
metal-layer is hidden outside of Ghost.

The Ghost-XR graphics binding for DirectX is responsible for managing
the DirectX compatibility now.

Cleanup: Avoid complex template type for XR-Swapchain

I rather avoid types like
`std::vector<std::unique_ptr<GHOST_XrSwapchain>>`, which is easy to do
in this case.

Fix missing XR space destruction

I think destructing the XrSession would destruct this anyway, but rather
have this done explicitly, consistently and in a controlled manner.

Fix VR session toggle not changing reliably on session start/end

The text and icon were supposed to change but didn't reliably, which was
a race condition I think. It depended on how fast the OpenXR runtime
would transition the session state.
This also makes sure the correct notifier is sent on session exit.