import favourites, clear intent after usage, import waypoints from route...

- Allow to import favourites files via Intent
- When importing a gpx file, allow to import the file as waypoints (intermediates). Routes calculated by osmand can then be reused later on for routing.
- Fix for intent that got resent after 'back' or 'cancel'

Need to do some more tests, so not ready yet to merge. Let me know if you're ok with the idea/changes, and I'll carry out some 'real world' testing.

This allows e.g. to clean up tracks and make routes out of them. --> Import a track, show it, start navigation, export the navigation to file/mail and then import again with waypoints.