Editor: Fix remove_control_from_dock fails when dock is floating.

[zaevi]

Fixes #78878.

[zaevi]

set_window_enabled(false) will emit window_close_requested signal and the control will be back to editor's dock.

[zaevi]

A quick reproduction for #78878:

@tool
extends EditorPlugin

var btn : Button

func _enter_tree():
	btn = Button.new()
	add_control_to_dock(EditorPlugin.DOCK_SLOT_LEFT_BL, btn)

func _exit_tree():
	if btn:
		remove_control_from_docks(btn) # 
		btn.queue_free()
		btn = null

1. Enable the plugin and make it floating.
2. Disable the plugin. remove_control_from_docks will fail and the floating window will stay. (this PR fixes)
3. Close the floating window, causes crash.

[kitbdev]

Works as expected and code looks good.

[AThousandShips]

I believe this matches roughly what I attempted for this, will do some testing of this when I'm able

[KoBeWi]

CC @trollodel

[akien-mga]

Thanks!