[buteo-mtp] Fix multi-core builds.

Specify proper dependencies and things will work out OK.
[buteo-mtp] Remove unused battery level/subscriber.
Merge pull request #13 from rburchell/master

Fix multicore builds and remove unused battery level stuff.
Merge pull request #14 from amtep/master

Fix use of QMutex for Qt5 mutex semantics
[xdg] Use correct XDG directories for storing files
[mtp] Change MTP home directory path to home
[mtp] Use mkpath instead of mkdir, make sure directories exist
[usb] Make iothread termination reliable