Separate UpdateStatusBar from LineView constructor
Fix UpdateStatusBar: keep object alive

Citing the matplotlib documentation:

The canvas retains only weak references to the callbacks. Therefore if a
callback is a method of a class instance, you need to retain a reference
to that instance. Otherwise the instance will be garbage-collected and
the callback will vanish.

http://matplotlib.org/1.3.1/users/event_handling.html#event-connections
Separate DrawLineElements from LineView
Separate DrawConstraints from LineView
Store 'envx','envy' in model.tw container
Generalize and refactor LineView

Generalized for arbitrary functions in the model.tw results array.
Functionality largely split to different classes.
Remove mirror-feature of y-plot

This feature is only useful for non-negative functions. For the sake of
optical consistency it is now dropped altogether.
Add X/Y view
Fix element drawings for negative functions
Use a darker green for y-curve
Make tab title more useful
Decrease alpha for line elements
Fix DrawConstraint to work in the presence of multiple views
Separate matching logic from model
Remove support for upper/lower bound matching

This was only implemented in one method and not accessible via the GUI.
It can be readded in the future, but then it should also be usable.
Support matching x/y
Revert "Fix DrawConstraint to work in the presence of multiple views"

This reverts commit ada3463b02c995d5435a3d9fa78033173a388ebb.

Since 4d7051d9257f98561901208c455da3ce2904726a DrawLineElements operates
on a Matching object which is specific to the same view.
Set frame title to model name
Merge branch 'xy-view'