Implement window expand functionality (#53)

This implements feature request #53, which is to have a button which
expands the window to the current pixel scale size of the current
page. The functionality is accessible through the scale drop-down menu
in the toolbar, and through a menu item in the "view" menu which is also
mapped to the keyboard sequence "control-e", (e for "expand", I guess).

Below are some screenshots of the feature.

![screenshot 1](https://sites.google.com/site/bl0ckeduserssoftware/atril1.png)
![screenshot 2](https://sites.google.com/site/bl0ckeduserssoftware/atril2.png)
![screenshot 3](https://sites.google.com/site/bl0ckeduserssoftware/atril3.png)