Clean up evaluation license logic

Instead of storing the application type as a uint, we use the enum provided
by QCoreApplicationPrivate. The former resulted in a few cases of wrong
logic where the values got mixed up, such as always printing the QtCore
console warning, even for GUI applications.

The qt_eval_is_supported function has been refactored to return enums instead
of magic values, to make the logic easier to read.

The same goes for qt_eval_days_left, which now only concerns itself with
the number of days left. qt_eval_is_expired() has been added to use for
easy checking of expiration date.

Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Change-Id: Ia0e85b2103f790a7e02e0d6e567a477b3145fcb9
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@digia.com>