FE/Qt: file header cleanups.
FE/Qt: Network/Update Managers stuff: Defines: Cleanup.
A IVirtualBox todo; file headers.
FE/Qt: Moving VBoxVersion into the separate file.
FE/Qt: 5985: Update Manager should check for the new version again the same day after installing other than current VBox version.
FE/Qt: Rework new version notifier to be a part of new UIUpdateManager class. Implement Extension Pack updater (new version check + download + update).