DO NOT MERGE: Implemented Geometry-Node View Buttons

Background
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This pull request implements the new view toggling buttons for ```Geometry``` and ```Node``` views. As seen in the image below, the buttons are aligned to the bottom-right corner of the workspace view.

There are now three ways to toggle between these views:

* Through menu item ```View > Background 3D Preview > Navigate Background 3D Preview```
* Through shortcut key ```CTRL + G```
* Through the view buttons that are on the canvas

![geometry-node-view-buttons](https://cloud.githubusercontent.com/assets/5086849/3214326/8ee1429e-efac-11e3-90dd-9ebe7154efbc.png)

Testing Scenarios
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Since this is UI-only change, the following scenarios should be tested manually:

* Toggling between views done through the above three ways should properly update the button visuals
* Click-drag-release should not toggle view changes if the mouse is released outside of the button
* The current view (geometry vs. node) should persist when switching between ```Home``` and ```Custom``` workspaces