Studio: backgrounds dropped during the game are picked up by the Resources button

Layer gets backgrounds/; StudioBackgrounds re-reads the file list and the background
cache and shows the current background (with _depth/_albedo) anew. The Resources
button sits on the transport bar (every tab); a new scene picks its background from
a list with the source mod. studio_live covers drop, reload, replace and export.

scenes/studio_panel.tscn

@@ -126,6 +126,10 @@ layout_mode = 2
 size_flags_horizontal = 3
 mouse_filter = 2
 
+[node name="ReloadButton" type="Button" parent="Frame/Dock/Column/Transport/Row"]
+layout_mode = 2
+focus_mode = 0
+
 [node name="RestartButton" type="MenuButton" parent="Frame/Dock/Column/Transport/Row"]
 layout_mode = 2
 focus_mode = 0