Please make abstreet package-friendly

[yurivict]

Data can be pre-downloaded and installed into a read-only location by the package, and can also be downloaded in the runtime into the user's home directory.

Currently the application doesn't seem to understand that the location is read-only and breaks with "something wrong" message: #803

When no data is present - the application just fails with 'No such file or directory': #802

It should always offer the user to download data into the home directory instead of into the installed location. And this should be the only option when the installed location is read-only.

[dabreegster]

I've responded to the individual issues with some ideas for fixes. I think the main idea is to further detangle the data/system directory. The FreeBSD package shouldn't even include data/system/us/seattle -- by default, it should ship without any city's data at all. That requires first making some of the different app title screens start on a blank map and first prompt the user to choose a city to download.

I can't promise a timeline for working on these issues, but I'll get to them as quickly as I can.