gui: introducing multiple detector streams

MDStreams allow to represent the acquisition from multiple detectors at the same time.

work in progress
UI margin tweak
gui: changed number formatting functions so they include an optional parameter with which to designate the number of significant decimals.
gui: Added selection size label
gui acquisition: handle multiple detector stream in acquisition

also modify the API of the acquisition manager to compute the thumbnail separately from the acquisition
Merge branch 'master' of vpn.delmic.com:odemis