Feature: backups & track sharing

[liinahamari]

• explore Storage Access Framework functionality and file access of it in particular;
• investigate .gpx file format of OpenStreetMap, it's impact and tracks' sharing guidelines;
• provide to end user options of creating, saving, updating, deleting and sharing* tracks;
• test saved in the App tracks in other applications supporting .gpx files (such as OsmAnd).

* sharing of traces is optional for now, the separate ticket is needed on demand.