Add back Benchmark dialog

We lost this useful feature with the rewrite and a lot of people
seemed to be sad about that. So here it is again, with a couple of
improvements

 - Everything is done in the gnome-disks process
   (we get the fd via udisks' just-added Block.OpenForBenchmark() method)

 - The graph is updated while benchmark is in progress

 - Write-benchmarks doesn't destroy data (same data is written back)

 - You can benchmark volumes/partitions as well as drives

 - The user can configure number of samples as well as the sample size

Require udisks master for this to work.

Screenshots:

 http://people.freedesktop.org/~david/disks-benchmark-1.png
 http://people.freedesktop.org/~david/disks-benchmark-2.png



Signed-off-by: David Zeuthen <zeuthen@gmail.com>