Summary:
- According to Copyright and license in Openstreetmap
(https://www.openstreetmap.org/copyright/en)
credit has to be in the corner of map.
- Add copyright_cb which returns an object to show copyright.
Test Plan:
1. Excecute elementary_test->map
2. Right click->source->set any tile source
3. Observe copyright is added.
Reviewers: cedric, Hermet, jpeg
Subscribers: conr2d
Differential Revision: https://phab.enlightenment.org/D4449