Adding kiosk mode support for mac and gtk targets

Adds a `--kiosk` option to start exo_browser in fullscreen mode.
Implemented for GTK and Cocoa.
Tested for GTK but **NOT TESTED** on mac.

Needs a CR and testing on MAC. The options passed might be more restrictive than we need (i don't know how chrome behaves in kiosk mode on mac). I quickly put it together following [this link](https://developer.apple.com/library/mac/technotes/KioskMode/Introduction/Introduction.html).