[TreeProjectWidget] Add Show/Hide Filesize Info, PEP8 Fixes

- Add Show/Hide Filesize Info to TreeProjectWidget context menus.
- Add new method to Show/Hide filesize information on QTreeView.
- PEP8 Fixes.
- Lint Ok.

![temp](https://cloud.githubusercontent.com/assets/1189414/2532830/6bc23494-b54b-11e3-8d91-6dbd69e27cd7.jpg)

Brancho: feature/treeproject_filesizeinfo  :8ball: