Make default pane indicator internal

No need to expose this.  There's a lot of copy/paste between
implementations of IPaneIndicator, this should be refactored at some
point.
Merge pull request #183 from odalet/development_3.0

VS2012 Theme enhancement (in Demo project)
Fixed document icon display issue.
Fixed #175 by accepting the patch from @lukeer
Fixed #161.
Fixed #172.
Updated release notes.
Fixed ActiveDocument property.
Merge pull request #219 from nabeshin/hotfix/active_document

Fixed ActiveDocument property.