Some action callbacks are already guarded with !NEMO_IS_DESKTOP_WINDOW()
checks, but not enough unfortunately.  Guards are added to these
callbacks for the following justifications:

CTRL+Q: action_close_all_windows_callback (segfaults)

CTRL+L: action_menu_edit_location_callback (does nothing but print
nonfatal assertion failures)

CTRL+D: action_add_bookmark_callback (adds bogus bookmark)

CTRL+B: action_edit_bookmarks_callback (crashes nemo-desktop with fatal
assertion failure)

ALT+P: action_plugins_callback (fails to load extensions)

F9: action_show_hide_sidebar_callback/nemo_window_get_sidebar_id
(silently changes global sidebar preference even though desktop has no
sidebar)

/ or ~: nemo_window_prompt_for_location (does nothing but print nonfatal
assertion failures)