MediaPlayerThread is a 30fps background thread that was calling GTK
widget methods directly, violating GTK's single-thread requirement.
C-level memory corruption from the race caused a SIGSEGV ~7 seconds
after startup (after the main window maps and image updates begin).

Wrap the three offending calls in GLib.idle_add() to schedule them
on the main loop thread:
- ControllerKey.set_ui_key_image() in DeckController.py
- ControllerTouchScreen.set_ui_image() in DeckController.py
- ScreenBarImage.set_image() in ScreenBar.py (sidebar icon selector)

Fixes StreamController#566

Three more GTK threading violations found after further testing:
- LabelManager.set_action_label() called update_label_editor()
  unconditionally even when reached from plugin rpyc threads
- LayoutManager.update() called update_layout_editor() directly
- BackgroundManager.update() called update_background_editor() directly

All three functions call GTK widget methods (label_editor,
image_editor, background_editor load_for_identifier()) that must
run on the main loop thread.

Wrapping with GLib.idle_add() fixes the remaining SIGSEGV triggered
on page-load events after extended runtime.

Page.initialize_actions() is called from a background thread during
load_page(). It called load_initial_generative_ui() synchronously,
which invoked ComboRow.set_selected() and other GTK widget methods
from that background thread — corrupting GtkListItemManager state
and causing an abort with:
  Gtk:ERROR:gtklistitemmanager.c:1039:gtk_list_tile_split:
  assertion failed: (n_items < tile->n_items)

Route the actual widget initialization through GLib.idle_add() so
it runs on the GTK main loop thread. The public method schedules the
work; a private helper performs it.