Removal of 'fmtc_plus_background_downloading' & deprecation of 'fmtc_plus_sharing' in v9

[JaffaKetchup]

This is announcement of intent to stop maintaining the 'fmtc_plus_background_downloading' & 'fmtc_plus_sharing' packages.

Background downloading functionality will not be integrated into flutter_map_tile_caching core. Background downloading was semi-functional on many devices (at best), and only supported Android through outdated and undermaintained packages. Therefore, it's functionality will not be upgraded to v9. Should better packages arrive in future, this decision may be reversed.

Import/export functionality will be integrated into flutter_map_tile_caching core, although without the UI features.

This decision has been made to reduce the cost of maintainence, and allow me to better focus on maintaining FMTC core, and the flutter_map project.

If you have any questions, please feel free to get in touch on the flutter_map Discord server (#plugins channel).