The shell design says that upon launching an application,
no X window should have focus, and we should display an
animated launching indicator.
Implement this by in panel.js, keep track of the last started
application. If there isn't currently an X focus, show an animation
for the last starting application.
https://bugzilla.gnome.org/show_bug.cgi?id=598349
7f8f0f2358
