Recent files

I have realized Recent Files submenu in Qucs main window. Now you can open recent qucs schematics from this menu, or recent openned docs list. Recent files list is saved in QSettings. I hope it will be usefull.
![new-menu](https://f.cloud.github.com/assets/4920080/1508416/fe47b204-49f7-11e3-91fb-ac4d3ddccf19.png)