Enforce proper MVVM architecture

[zspitz]

Relates to #22

Also important if we want to create a separate visualizer on token streams. We'll have to tease apart the token list control and model into a reusable form.

[zspitz]

We still need to separate models from viewmodels.

[zspitz]

For each class, split the model and viewmodel parts into two separate classes, both directly under the ParseTreeVisualizer namespace, but in separate folders.

[zspitz]

Further goals:

• Abstract base class that handles selection and notification: IsSelected property, with calls to PropertyChanged
• Should WatchBaseExpression be a static field?

[zspitz]

There are still some residual bugs with the selection. Currently:

• clicking on one or more tokens correctly modifies the selection in both the treeview and the source textbox
• clicking on a node in the treeview correctly modifies the selection in both the source and the token list

However, clicking on the source, while it correctly selecting the corresponding node in the treeview and the corresponding token(s) in the token list, has the following bugs:

• intermittently, the initial click doesn't seem to change the selection in the other parts, only on the second click
• the cursor isn't visible when only one character is selected
• double-click has some very odd behavior

We need first to implement a test project for the viewmodel ( #33 ) after which we can pin down if the problem is in the viewmodel or in the textbox.

[zspitz]

All the selection bugs appear to have been resolved, except for #42