Remove OnlyShowIn from desktop actions

The OnlyShowIn key is not supported in these sections and ignored
by gnome-shell.
Don't use deprecated GTK+ API

This commit drops gtk_window_get/set_opacity.
Don't use deprecated GTK+ api, II

This commit drops gtk_button_new_from_stock.
Don't use deprecated GTK+ api, III

This commit drops gtk_image_new_from_stock.
Don't use deprecated GTK+ api, IV

This commit drops stock icon registration.