Notification handlers now use LLHandles to screen channels.

Fixed boost trackable objects usage.

As far as I can tell, if login fails with the "Region has begun the logout process" message, gAgentAvatarp gets destroyed, which leads to the crash.
Adding a check that should prevent it (could not reproduce to make sure).

Added filtering empty folders as any other folder view item.
This enables dragging empty folders between Inventory floaters when they pass the filter.

- Dragging folder into Recent Items disabled, but allowed within Recent Items.
- Dragging Contents folder from task inventory disabled.
- Filtering folder contents upon dragging to any inventory panel added, to make the behavior consistent with items filtering during DnD.