* prevent crash if trying to load non existent calendar (closes #17)

* Minor changes

Co-authored-by: Ricki Hirner <hirner@bitfire.at>

* give proper error message when a synced file is deleted (closes #19)

* extract strings to xml

* Also handle FileNotFoundException (for instance thrown by Google Drive when a file is not there anymore)

* Require context for taking permissions

Co-authored-by: Ricki Hirner <hirner@bitfire.at>

Add starting framework for tests. Closes #10

Co-authored-by: Ricki Hirner <hirner@bitfire.at>

Support Storage Access Framework

Co-authored-by: Ricki Hirner <hirner@bitfire.at>

Co-authored-by: Ricki Hirner <hirner@bitfire.at>

See also https://github.com/bitfireAT/davx5-ose/blob/dev-ose/CONTRIBUTING.md