Open favorites at startup and re-activation

Added a checkbox in the files preference pane which enables showing the
favorites chooser at startup and re-activation with no open documents.
This option is mutually exclusive with «Create one at startup» and
«Create one at re-activated» settings.

![screenshot 2014-12-25 23 36 17](https://cloud.githubusercontent.com/assets/1584968/5554875/2b112bfa-8c8f-11e4-8ccf-11e8fd55f30e.png)