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/breakpoint.c

@@ -53,6 +53,7 @@
 #include "memattr.h"
 #include "ada-lang.h"
 #include "top.h"
+#include "ui.h"
 #include "valprint.h"
 #include "jit.h"
 #include "parser-defs.h"