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/python/python.c

@@ -64,6 +64,7 @@ static const char *gdbpy_should_print_stack = python_excp_message;
 #include "cli/cli-decode.h"
 #include "charset.h"
 #include "top.h"
+#include "ui.h"
 #include "python-internal.h"
 #include "linespec.h"
 #include "source.h"