Initial working version of model/view based variable viewer.
- ported to model/view: new class VariableCollection, owned by the controller, doubles as the model
- new abstract base class AbstractVariableItem replaces TrimmableItem
- reworked implementations of subclasses; moved MI command requests into subclasses + away from viewing widget
- currently disabled special views for variables like QString; I plan to create a new interface for special view plugins to be able to make this much more
customizable (including things like easy access to items in Qt's tulip classes)

Current breakage includes:
- a new toplevel item is created for each thread/frame. It seems to be a feature of the previous code that got removed - was this accidental or not? Anyway,
still need to make the latest frame auto-expand
- not very heavily tested (only about 2 hours of testing so far)

Other miscellaneous small changes
- move event_t from controller to global namespace
- remove newlines from debug statements
- fix access to out of bounds characters in strings
- remove some deprecated calls