gtk: add separate close_window apprt action

For *some* reason we have a binding for close_window but it merely closes
the surface and not the entire window. That is not only misleading but
also just wrong. Now we make a separate apprt action for close_window
that would make it show a close confirmation prompt identical to as if
the user had clicked the (X) button on the window titlebar.

include/ghostty.h

@@ -600,6 +600,7 @@ typedef enum {
   GHOSTTY_ACTION_COLOR_CHANGE,
   GHOSTTY_ACTION_RELOAD_CONFIG,
   GHOSTTY_ACTION_CONFIG_CHANGE,
+  GHOSTTY_ACTION_CLOSE_WINDOW,
 } ghostty_action_tag_e;
 
 typedef union {