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..