Fixes to 2D viewport

[reduz]

• Editor 2D viewport now uses embedded subwindows (windows no longer pop up)
• Restored the ability to disable 3D on the 2D viewport (makes 3D not display on 2D when there is a camera on the scene)

Bugsquad edit: Fixes #40553

[Calinou]

Restored the ability to disable 3D on the 2D viewport (makes 3D not display on 2D when there is a camera on the scene)

We may want to expose an option in the 2D editor for this, since seeing a 3D background can sometimes be useful for designing HUDs in real world conditions. This can be exposed in a future PR.

[KoBeWi]

Editor 2D viewport now uses embedded subwindows (windows no longer pop up)

Is it supposed to fix dialog display? Dialogs don't appear at all and windows are like this

[reduz]

@KoBeWi This is mostly because it does not have anything to draw and there is no default theme for embedded windows. Additionally, subwindows/dialogs are not editable(I don't think they ever were). Would be nice if they were but likely all this is material for a separate PR.

[akien-mga]

Thanks!