Add property to enable "Launch app at exit" by default.

All the logic and instrumentation to launch Mumble by default upon successful install is already in the installer. All that's missing is the property whose existence is checked.
 
This patch adds this property, which will allow Mumble to automatically restart itself upon an upgrade.
 
Only enabling the checkbox for an upgrade rather than a fresh install would be the subject of another patch.

(Sourceforge tracker artifact ID: 3192385)