Improve editor 2D/3D main screen auto-switching logic

[aaronfranke]

In master, Godot has a feature where the main screen tabs may auto-switch depending on the node type being edited.

• When editing a 2D scene or selecting a 2D node, and the 3D tab is selected, switch to 2D.
• When editing a 3D scene or selecting a 3D node, and the 2D tab is selected, switch to 3D.
• When creating a new 2D or 3D scene, and the tab isn't the script editor (2D/3D/Game/AssetLib/custom), switch.

Seems simple enough. However, the code currently has the following problems:

• There is a hard-coded assumption that the script editor is at index 2.
• Instead of using func _handles on each plugin, there is hard-coded logic to check for CanvasItem and Node3D.
• The logic does not behave the same when creating scenes as it does when editing scenes or nodes.
• There is some dead code in the script editor for deciding when to switch (it always returns true, but never happens).

main_screen_switch_current_master.mp4

This PR makes the following changes:

• New function bool EditorMainScreen::can_auto_switch_screens() const.
  • This is used both when editing nodes/scenes and when creating new ones, so now the logic is unified.
  • Instead of hard-coding an index for Script, check what position it's at. The new logic is that any tab to the left of Script is considered auto-switch-away-able, and auto-switch-to-able is determined by func _handles.
  • For the built-in tabs, this does change behavior in one case: creating a new scene when either the Game or AssetLib tabs are active will keep you on those tabs, just like editing a scene already does.
• Set a human-readable unique name for the EditorMainScreenButtons HBoxContainer.
• Remove the dead can_take_away_focus and can_lose_focus_on_node_selection functions.

This now works correctly for custom node types and main screen tabs placed to the left of Script:

main_screen_switch_with_pr.mp4