If stop_watching_signal is called on a signal that isn't being watched, ignore

Previously it would assert.  Really we should switch the api to have a signal_watch
object much like the fd_watch object we have for watch_fd.

TODO

@@ -10,3 +10,4 @@
 - fix error handling.  In particular, ply_open_module gets it completely wrong (replies on errno instead of dlerror())
 - consider moving text code from ply-window to a ply-text-buffer analog of ply-frame-buffer
 - maybe watch for VT switches and do things like restoring text color palette and stopping drawing the framebuffer
+- clean up the event loop watch api to always be in terms of watch objects instead of function/user_data pairs.

src/libply/ply-event-loop.c

@@ -810,7 +810,8 @@ ply_event_loop_stop_watching_signal (ply_event_loop_t *loop,
   node = ply_signal_dispatcher_find_source_node (loop->signal_dispatcher,
                                                  signal_number);
 
-  assert (node != NULL);
+  if (node == NULL)
+    return;
 
   ply_signal_dispatcher_remove_source_node (loop->signal_dispatcher, node);
 }

src/main.c

@@ -176,6 +176,7 @@ static void
 on_hide_splash (state_t *state)
 {
 
+  ply_trace ("hiding boot splash");
   if (state->boot_splash != NULL)
     {
       ply_boot_splash_hide (state->boot_splash);
@@ -183,8 +184,10 @@ on_hide_splash (state_t *state)
       state->boot_splash = NULL;
     }
 
+  ply_trace ("closing splash window");
   if (state->window != NULL)
     {
+      ply_window_close (state->window);
       ply_window_free (state->window);
       state->window = NULL;
     }