gdb: move struct ui and related things to ui.{c,h}

I'd like to move some things so they become methods on struct ui.  But
first, I think that struct ui and the related things are big enough to
deserve their own file, instead of being scattered through top.{c,h} and
event-top.c.

Change-Id: I15594269ace61fd76ef80a7b58f51ff3ab6979bc

gdb/exceptions.c

@@ -26,7 +26,7 @@
 #include "ui-out.h"
 #include "serial.h"
 #include "gdbthread.h"
-#include "top.h"
+#include "ui.h"
 #include "gdbsupport/gdb_optional.h"
 
 static void