EULA: update the service code to avoid deadlocks

[ForNeVeR]

1. In certain cases, when showing a EULA dialog is impossible (the current thread holds a read or a write lock), we'll show a corresponding notification. An exception may still be thrown in those cases, but this shouldn't matter: after accepting the EULA, everything should start working correctly. Or, in the worst case, after accepting EULA and restarting the IDE.
2. There are new internal actions (-Didea.is.internal=true) to show the notifications (for QA purposes).
3. For cases when something's completely wrong and the user still cannot accept the EULA or see the notification, there are new non-internal actions: Show TF Client EULA, Show TF SDK Agreement. The users will be able to invoke them manually if required.
4. PropertyServiceImpl is now more thread-safe.

Closes #539.

[ForNeVeR]

Tests ok.