Checkbutton

Hi,

I've implemented checkbutton widget for lambda-term. Also added an example of resource file as one of the commits as I needed it anyway to see which button has the focus.

One thing to mention: I made checkbuttons switching their state on both Enter or space. This is a bit inconsistent with normal buttons which are activated only by Enter but I was hitting space all the time when testing checkbuttons and wondering why it does not work. :)

That's how the things look like (with resource file I've put into repository as a configuration example):

![2014-05-09 20 39 49](https://cloud.githubusercontent.com/assets/130091/2931249/1852ee2a-d7a1-11e3-9389-de0610c3cffd.png)