Add Migration functionality to D-Bus API

[Jason Gerard DeRose]

The file import UX design helped clarify the difference between Importing and Migration. The D-Bus API was formerly a muddy mixture of the two, not particularly good for either. lp:684460 refocused the existing API exclusively on Importing, which is what is needed for the file import UX design.

Unfortunately, we lost of Migration-like functionality, which needs to be restore. This is a pretty small addition and Migration will use most of the same Importing functionality behind the scenes anyway.

However, Migration merits a different UX. Unlike Importing, Migration will likely only be done once or twice ever. Migration merits a lot of control, migration might very well be something *do* want to babysit. My general thoughts on the UX:

1) Only one migration runs at a time
2) Migration opens a window, shows detailed progress (does not use notifications or rendermenu)
3) Lots of control over migration... migrate only certain file types, sizes, timestamps, whatever

This is also needed to restore functionality previously provided by lp:681866

[Jason Gerard DeRose (jderose)]

I'm trying to keep the 0.4 cycle more focused, so I'm re-targeted this for 0.5 as it doesn't fit the 0.4 theme. Which is not to say someone can't take this on during 0.4 - if you completed it, I will land it!