Fix Color Picking

[Sauermann]

With the 4.x-introduction of Windows the previous method for color picking was no longer working.

Resolve #68979

This PR uses the following approach to reimplement color-picking:

1. When the Color-Picking-Button is pressed, a screenshot of the Window-content is created and displayed in a new Popup-Window. (Areas outside of application Windows are interpreted as black color)
2. This new Window allows selecting colors by Mouse-Click. A Preview of the targeted Color is also displayed.

Other failed attempts to solve this include:

• Simple replacement of Control *screen by Popup *screen: failed because Popup is not reliably transparent (Platform DisplayServer limitation).
• Use Drag&Drop-mechanic: might be possible, but the Input-mechanic for this becomes very complex and in effect error-prone.

Alternative ideas for solving this issue are mentioned in #68979 (comment)

This PR currently has the following shortcomings, but they could be solved in later PRs:

• Windows need to be sorted by visibility.
• Non-default Transform2D of the Window can lead to wrong image-coordinates.
• The created image-texture could be reused in later Color-Picking operations.
• The created Popup-Nodes and Images linger in memory and could be removed after the Color-Picking operation.
• The created Popup and its content could be visually improved.
• Multiple Display Screens would likely need to be supported.

Updated: 2023-02-06: Fix also the embedded-use-case after #72409 got fixed
Updated: 2023-02-09: Fix merge conflict

[bruvzg]

Color picker probably needs a native API to get screen pixel added to the DisplayServer. This way it will be able to pick from other apps and avoid overcomplicated window sorting.

[KoBeWi]

I think it would be nice to have this PR merged, because I can confirm it fixes the issue (and picking is completely broken right now).

[akien-mga]

Thanks!