File list sidebar on Admin

This patch provides a simple sidebar for showing the file list while editing a page in admin. Improvements that can be added are a lot! Just to name a few:
- refactory, the side bar should be an ajax component, loaded dynamically.
- ajax support, a new page is loaded dynamically without reloading the whole page.
- warning alert, to block the loading of a new page if the current one has not been saved yet.
- Latest on top, to put above the list a list of the most recent edited files.
- better sub-directory handling. 
- ...

Hope it helps.
Paolo