Made the MountWatcher update state for remotes located in a drive that
gets mounted. This was tricky code. First I had to make remotes declare
when they're located in a local directory. Then it has to rescan git
configs of git remotes (because the git repo mounted at a mount point may
change), and update all the state that a newly available remote can affect.

And it works: I plug in a drive containing one of my git remotes, and the
assistant automatically notices it and syncs the git repositories.

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But, data isn't transferred yet. When a disconnected remote becomes
connected, keys should be transferred in both directions to get back into
sync.

To that end, added Yet Another Thread; the TransferScanner thread
will scan newly available remotes to find keys, and queue low priority
transfers to get them fully in sync.

(Later, this will probably also be used for network remotes that become
available when moving between networks. I think network-manager sends
dbus events it could use..)

This new thread is missing a crucial peice, it doesn't yet have a way to
find the keys that need to be transferred. Doing that efficiently (without
scanning the whole git working copy) is Hard. I'm considering design
possibilities..