Initialize ShellAppMonitor/ShellAppSystem at predicatable time

Currently ShellAppMonitor relies on all the .desktop files being
loaded.  We should initialize it very early to ensure that anything
that uses it has up-to-date information right away.

js/ui/main.js

@@ -40,6 +40,16 @@ function start() {
 
     Tweener.init();
 
+    // Ensure ShellAppMonitor is initialized; this will
+    // also initialize ShellAppSystem first.  ShellAppSystem
+    // needs to load all the .desktop files, and ShellAppMonitor
+    // will use those to associate with windows.  Right now
+    // the Monitor doesn't listen for installed app changes
+    // and recalculate application associations, so to avoid
+    // races for now we initialize it here.  It's better to
+    // be predictable anyways.
+    Shell.AppMonitor.get_default();
+
     // The background color really only matters if there is no desktop
     // window (say, nautilus) running. We set it mostly so things look good
     // when we are running inside Xephyr.