[client] wayland: implement minimize operation

This commit implements minimize for both xdg-shell and libdecor
pseudo-shell. This fixes #535.

client/displayservers/Wayland/shell_xdg.c

@@ -149,5 +149,5 @@ bool waylandGetFullscreen(void)
 
 void waylandMinimize(void)
 {
-  //FIXME
+  xdg_toplevel_set_minimized(wlWm.xdgToplevel);
 }