View contextual menu enhancements

Background
-----
This pull request enhances the existing canvas contextual menu to include view manipulation items (e.g. pan and rotate). The main idea is to make these view operations more obvious to beginners of Dynamo when they first use it. The pull request implements what is described in the following task tracked internally:
- [MAGN-3675](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-3675) Implement view switching command on canvas contextual menu

![view-context-menu-both](https://cloud.githubusercontent.com/assets/5086849/3465125/30b1e3c6-025e-11e4-8776-6f07f62534bf.png)

Testing Scenarios
-----
Here are few things to look out for while testing these additions:
- Only ```Pan``` and ```Orbit``` operations can be toggled (i.e. they can be turned on/off and stay that way).
- When ```Pan/Orbit``` is active, the corresponding menu item should be ```checked```
- When ```Pan/Orbit``` is active, the corresponding canvas button (those on top-right corner) should be in blue color
- Pressing ```Escape``` key cancels the current ```Pan``` or ```Orbit``` operation

Side Notes
-----
There are several things to note with this pull request:
- ```Watch 3D``` nodes do not serve up the same contextual menu as in ```Geometry View```, but regular gestures to ```pan``` (middle mouse button) and ```rotate``` (right mouse button) are still applicable. The ```Zoom to Fit``` menu item remains available.

- This pull request also removed some unused files from the solution:

```
- src/DynamoCore/UI/Views/WatchViewFullscreen.xaml.cs
- src/Libraries/DynamoWatch3D/Watch3DFullscreenViewModel.cs
- src/Libraries/DynamoWatch3D/WatchView.xaml.cs
- src/Libraries/DynamoWatch3D/WatchViewFullscreen.xaml
- src/Libraries/DynamoWatch3D/WatchViewFullscreen.xaml.cs
```

Hello @ikeough, @pboyer, please help me to take a look at these changes. Thanks :)