Keep the bottom panel visible when enabling the distraction-free mode

- Document the `EditorInterface.distraction_free_mode` property.

This closes https://github.com/godotengine/godot-proposals/issues/951.

(cherry picked from commit e6352d1daa)

doc/classes/EditorInterface.xml

@@ -204,6 +204,7 @@
 	</methods>
 	<members>
 		<member name="distraction_free_mode" type="bool" setter="set_distraction_free_mode" getter="is_distraction_free_mode_enabled">
+			If [code]true[/code], enables distraction-free mode which hides side docks to increase the space available for the main view.
 		</member>
 	</members>
 	<constants>

editor/editor_node.cpp

@@ -4296,7 +4296,6 @@ void EditorNode::_update_dock_slots_visibility() {
 		}
 
 		right_hsplit->hide();
-		bottom_panel->hide();
 	} else {
 		for (int i = 0; i < DOCK_SLOT_MAX; i++) {
 
@@ -4326,7 +4325,6 @@ void EditorNode::_update_dock_slots_visibility() {
 				dock_slot[i]->set_current_tab(0);
 			}
 		}
-		bottom_panel->show();
 
 		if (right_l_vsplit->is_visible() || right_r_vsplit->is_visible())
 			right_hsplit->show();