Fixed trailing / in ~/ expansion.

If ~/ was expanded the path had a trailing / but according to the shell this
should end up in directory without a slash.
Save current uri in property for easier access.
Added first unit tests.
Remove obsolete GET_URI() macro.

Removed also the changed-uri signal observer. We get the new URI in the load
status callback right before the event is triggered.
Write VIMB_URI environment variable.