Warn if download overwrites user-added wifi catalog ('synced locally')

[wish7code]

Currently server wifi catalog always overwrites any local wifi catalog. Thus all local synced wifis are overwritten (Settings --> Sync locally). This isn't a issue at all if user has uploaded all of it's scans, but may cause some unwanted behaviour if user hasn't uploaded his tracks before.

Suggestion: display warning before download: "You're wifi catalog has local changes. Would you like to overwrite your local version?"

[jw243]

Maybe it would be possible to change the download function such that it only downloads new datasets and adds them to the local database. Since the size of the database has become huge, it is quite a pain to download the whole database again.

Otherwise, splitting the database into much smaller regional databases would be a solution.

[gdt]

Separately from splitting, I wonder if having a separate local database is sensible. That then makes me wonder about integration with the local wifi NLP module, since it's sort of doing the same thing. Essentially, have the local module only store things not in the downloaded openbmap database, and have local sync add to local db instead. This does blur the "localwifi has no network" notion, and I'm not sure an intent per new wifi is sensible.