gdb: move a bunch of quit-related things to event-top.{c,h}

Move some declarations related to the "quit" machinery from defs.h to
event-top.h.  Most of the definitions associated to these declarations
are in event-top.c.  The exceptions are `quit()` and `maybe_quit()`,
that are defined in utils.c.  For consistency, move these two
definitions to event-top.c.

Include "event-top.h" in many files that use these things.

Change-Id: I6594f6df9047a9a480e7b9934275d186afb14378
Approved-By: Tom Tromey <tom@tromey.com>

gdb/dwarf2/cooked-index.c

@@ -24,6 +24,7 @@
 #include "cp-support.h"
 #include "c-lang.h"
 #include "ada-lang.h"
+#include "event-top.h"
 #include "split-name.h"
 #include "observable.h"
 #include "run-on-main-thread.h"