Which directory to watch

[rschroll]

Currently, Beru watches ~/Books for epub files. But people may already have their books stored elsewhere (~/Documents/Books, for instance), and we should be able to respect that somehow. A command line option would work on the Desktop, but not be so hot for a phone.

Some additional wrinkles:

• If running with restricted access to the file system, we may not be able to watch another directory, even if the user requests it.
• Should the default location be localized? Presumably, but we run into issues with requesting access, as above.
• Once we get the Content Hub, this all becomes moot. I think.

[rschroll]

e820838d adds a setting for this. Of course, it doesn't work under confinement.