core: add an ANR dialog

for xdg-shell, we can ping the wm_base, and thus render an ANR dialog if an app dies

for XWayland, there probably is a similar method, but I don't know about it and don't care.

src/protocols/XDGShell.hpp

@@ -241,12 +241,17 @@ class CXDGWMBase {
 
     bool                                    good();
     wl_client*                              client();
+    void                                    ping();
 
     std::vector<WP<CXDGPositionerResource>> positioners;
     std::vector<WP<CXDGSurfaceResource>>    surfaces;
 
     WP<CXDGWMBase>                          self;
 
+    struct {
+        CSignal pong;
+    } events;
+
   private:
     SP<CXdgWmBase> resource;
     wl_client*     pClient = nullptr;