PM: cancel build button (closes #184) + context menu improvements

This turns the "Build Project" buttons / items into "Cancel Build" while building. This approach saves UI space and seems rather intuitive.

![](http://i.imgur.com/ZDh4AvD.png)

![](http://i.imgur.com/Pu60A3M.png)

Additionally, this has the following consistency fixes for the context menu of the PM panel:

- Items are now disabled like elsewhere while building
- Added "Run Project" and "Clean Project"

![](http://i.imgur.com/v1uhWz9.png)