Make the check for configured app dynamic

[glyptostroboides]

A user experienced an issue when desinstalling a software that appear in the "Preferred app" list.
The UI should take care of the installed software and so not depend on the configuration file : bumblebee.cfg which is only modifyed by app-settings.
The list of preferred app has to be determine dynamically when the mouse get over the "Preferred apps" submenu.