Always run async signal handlers in the main UI
    
    Async signal handlers have no connection to whichever was the current
    UI, and thus always run on the main one.
    
    gdb/ChangeLog:
    2016-06-21  Pedro Alves  <palves@redhat.com>
    
    	* event-loop.c: Include top.h.
    	(invoke_async_signal_handlers): Switch to the main UI.
    	* event-top.c (main_ui_): Update comment.
    	(main_ui): New global.
    	* top.h (main_ui): Declare.