Callbacks in timeouts called without GDK lock

Callbacks in the overlay scrollbar that are called by timeout are setup by calls
to g_timeout_add, which means that the callbacks are called without GDK lock. This
is a problem because most (or all) of them do GTK operations that require the
GDK lock.

Instead of g_timeout_add, it should use gdk_threads_add_timeout. I didn't
carefully check all the code, but I think all calls to g_timeout_add need changing.

If the code use g_idle_add anywhere, it may need to be fixed (I didn't see any).

We found it because we have multi-threading application, and when using
overlay scrollbar it randomly crashes, so we checked and found it.

We found about LIBOVERLAY_SCROLLBAR, so we set it to 0 now.