Edit Mode GUI as a ToolbarOverlay

[SixParQuatre]

The changes makes it that, if the project is using Unity 2021 or newer, the Edit Mode GUI is not rendered and an OverlayTab is shown instead. Tested in 2020.3, 2021.3 and 2022.3. Though in 2021, the defaultDisplay flag to show the OverlayTab in new Scene editor, doesn't seem to work.

I'd argue this is a good first step to submit vs. waiting for all tool GUI to be ported to the OverlayTab: it makes the first time UX a lot more digestible and remove the conflict of the old UX that was stuck in a corner and was displayed below the Overlay for general manipulation.

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Added:

• EditorModeOverlay: a ToolbarOverlay that offers the same functionality as the Edit Mode GUI found before; plus a button to toggle Realtime CSG On/Off.
• Icons for each edit mode Modified
• CSGSettings: added a static Action invoked when EnableRealtimeCSG value changes.
• EditModeManager: Added an Action invoked when the EditMode changes. Added a static property to access it from other classes.
• EditModeSelection.GUI: surrounded the display of the former Area by #if !UNITY_2021_3_OR_NEWER so it isn't displayed beyond this version
• CSG_GUIStyleUtility: move the initialization of BrushEditModeContent and brushEditModeTooltips in a separate static function, so they can be called separately and used from EditorModeOverlay.

[SixParQuatre]

@nukeandbeans so this is my first time going though a pull request process, is there anything I need to do on my end at this point?

[nukeandbeans]

Hey! Apologies on taking a bit to get back to you. There isn't much for you to do yet. I need to test and verify it doesn't break things in branches of existing projects, though I'm not completely certain how soon I'll be able to at the moment. Your work so far looks fantastic, though! Thanks for the PR!

[SixParQuatre]

Hello and happy new year- any update on this ?

[SixParQuatre]

@nukeandbeans Another ping on this, just in case.