Redesign Asset dock for Godot 4.6

[TokisanGames]

Fixes #921
Fixes #931

godotengine/godot#106503 (presumably) broke the asset dock by improperly deprecating the old functions and breaking them, and redesigning new functions.

This PR make a 4.6 compatible asset dock and switches between the new in 4.6+ and old in <=4.5.

Pre 4.5 the editor docks were quite clunky and I wrote a lot of code to handle it's quirks. In 4.6 the system has been redesigned and things that didn't work now do. We no longer need our own dock selector as the built in one finally works properly.

Pending:

• 4.6 and 4.5 dual compatibility
• Search filter
• Search/pin getting icon from editor
• Pin button missing
• Verify update_layout() child ordering
• Instancer Asset icons going too small and crowding buttons
• Search box too narrow
• Always on top pin for window mode doesn't work (from pin)
• Review more code that needs to be removed or adjusted for slot handling or windowing
• Testing

Cherry-pick for 1.0.2