Remove Client::window() method, use a force_redraw() one

We cannot assume Client::m_window is always non-null, as when
changing current buffer its temporarily null, at the point
where WinCreate hook might get called.

Fixes #382

src/client.cc

@@ -166,6 +166,12 @@ void Client::redraw_ifn()
     ui.refresh();
 }
 
+void Client::force_redraw()
+{
+    if (m_window)
+        m_window->force_redraw();
+}
+
 void Client::reload_buffer()
 {
     auto& buffer = context().buffer();

src/client.hh

@@ -35,10 +35,10 @@ public:
 
     void print_status(DisplayLine status_line);
 
+    void force_redraw();
     void redraw_ifn();
 
     UserInterface& ui() const { return *m_ui; }
-    Window& window() const { return *m_window; }
 
     void check_if_buffer_needs_reloading();
 

src/commands.cc

@@ -1536,7 +1536,7 @@ const CommandDesc face_cmd = {
         FaceRegistry::instance().register_alias(parser[0], parser[1], true);
 
         for (auto& client : ClientManager::instance())
-            client->window().force_redraw();
+            client->force_redraw();
     }
 };