Relax splash on top

While we possibly spend a lot of time (like in xmp crawler ...) while having the splash screen open
users might want to do other work but the splash requested to be "above" prohibits that.

So let us **not** interfere in WM/user actions.

If we don't enforce the splash via preferences we close it right after the the crawler has done it's noteworthy
work.

src/common/darktable.c

@@ -1618,6 +1618,8 @@ int dt_init(int argc, char *argv[], const gboolean init_gui, const gboolean load
       darktable_splash_screen_create(NULL, TRUE); // force the splash screen for the crawl even if user-disabled
       // scan for cases where the database and xmp files have different timestamps
       changed_xmp_files = dt_control_crawler_run();
+      if(!dt_conf_get_bool("show_splash_screen"))
+        darktable_splash_screen_destroy();
     }
   }
 

src/gui/splash.c

@@ -257,7 +257,6 @@ void darktable_splash_screen_create(GtkWindow *parent_window,
   gtk_window_set_decorated(GTK_WINDOW(splash_screen), FALSE);
   gtk_widget_show_all(splash_screen);
   gtk_widget_hide(GTK_WIDGET(remaining_box));
-  gtk_window_set_keep_above(GTK_WINDOW(splash_screen), TRUE);
   _process_all_gui_events();
 }