FE/Qt: Precompiled header hacking (work in progress).

Frontends/VirtualBox: remove bogus assertion, the handling as such was OK

FE/Qt: Fix for r90911: Snapshot can be NULL.

FE/Qt: Medium enumeration mechanism: Full snapshot-tree update for every medium related to particular VM on machine registration event.

FE/Qt: Medium enumeration mechanism: Full update for every (cached and actual) medium related to particular VM on snapshot delete operation.

FE/Qt: Medium enumeration mechanism: Unify/reuse handling code used for different Main events.

FE/Qt: Separate Qt signals for snapshot take/delete events.

FE/Qt: Medium enumeration mechanism: Separate handling for machine registration/unregistration routines (it involves all the used mediums, not only attached to current state).

FE/Qt: Medium enumeration mechanism: Support to async CMedium close/delete logic.

FE/Qt: Medium enumeration mechanism: Cleanup.

GUI: Prevent cleanup race in VBoxGlobal media-enumerator-mess from crashing again during termination.

FE/Qt: Medium-enumeration feature: On machine state/data change: Invalidate only mediums related to old/new *current* state.

FE/Qt: Medium-enumeration mechanism: Some coding-style cleanup.

FE/Qt: Linguist thought that some comments were meta-data. Fixed here.

FE/Qt: Medium-enumerator: Some logging fixes.

FE/Qt: 6909: Medium-enumeration cleanup/rework (part 10): Switch to new medium-enumeration mechanism.

FE/Qt: Build-fix for r88688.

FE/Qt: Medium-enumerator framework.