core: file dropping: on compatible OSes, one or more file(s) can now be dropped on the main window, it will then be transmitted to the selected widget if it has a "on_drop_file" method. This is used in chat widget to add an attachment

-e git+https://github.com/kivy/kivy.git#egg=kivy
-e hg+https://repos.goffi.org/sat#egg=sat
-e .