window_manager: fix dangling empty window bug

Previously, some program might unmap their window(s) but didn't
destory them, leaving some empty windows in the workspace.

Example:
1. KDE's systemsettings5 -> Font -> Font selection window
2. WPS office sign in window

Some empty windows can't even be closed, so initially I simply
use XDestroyWindow to brutally close them, and I broke my entire KDE
config, lots of GUI applications segfaults.

A safer way I came up with is to send an X Event with WM_DELETE to close
a window, call XSync (to make sure the event we just sent has been
processed) and then call XDestroyWindow to make sure this danling bitch is
really closed.

src/window_manager.cc

@@ -370,7 +370,7 @@ void WindowManager::OnUnmapNotify(const XUnmapEvent& e) {
   // by the user, then we will remove them for user.
   Client* c = it->second;
   if (!c->has_unmap_req_from_user()) {
-    //c->workspace()->Remove(c->window());
+    KillClient(c->window());
   } else {
     c->set_has_unmap_req_from_user(false);
   }
@@ -680,6 +680,8 @@ void WindowManager::KillClient(Window window) {
     msg.xclient.format = 32;
     msg.xclient.data.l[0] = prop_->wm[atom::WM_DELETE];
     XSendEvent(dpy_, window, false, 0, &msg);
+    XSync(dpy_, false); // make sure the event we just sent has been processed by server
+    XDestroyWindow(dpy_, window); // make sure the window is really destroyed
   } else {
     XKillClient(dpy_, window);
   }