QbsProjectManager: Use tr() for translatable string.

Change-Id: I90bea6d1ef3e4584db5b5140d7ccb7b7c7ca529f
Reviewed-by: Tobias Hunger <tobias.hunger@theqtcompany.com>
QbsProjectManager: Put internal classes into the Internal namespace.

Change-Id: I5fa1ce2ed709b7904a0b2701133794751e5e2119
Reviewed-by: Tobias Hunger <tobias.hunger@theqtcompany.com>