Change Editable Children menu action to be keyboard shortcuttable

[miv391]

Fixes godotengine/godot-proposals#9229

Makes the Editable Children action in scene tree menu keyboard shortcuttable. No default shortcut is set.

[AThousandShips]

You also need to add this shortcut at line 3457

[KoBeWi]

Since the action had no shortcut, it never had a safeguard against invalid usage. Which means that you can use it on a node that isn't an instance and it will still toggle editable children as if it was an instance (even with a warning when disabling).

You need to add a condition in case the shortcut is used when the option is not available for selected node(s).

[miv391]

You also need to add this shortcut at line 3457

Fixed.

You need to add a condition in case the shortcut is used when the option is not available for selected node(s).

Fixed.

[akien-mga]

Thanks!