Purpose: to reduce dependency on the hopelessly flakey MTP interface for projects that can't afford a QFieldCloud subscription

For export: add an in-progress wheel so that the user knows when the project export is done.

For both: have each project remember which folder, if any it has previously imported to and exported from, and use that one by default next time (to reduce errors)

For both: have import and export move only changed files into the QField's storage, to reduce time needed for both these operations and phone-based synchronization programs