Qt: cohenrency for volume between FSC and main Controller. Solve the 1% issue reported by Trax, close #1851.

We need to fix that better in post 1.0.0
Close #2377

Qt: remove the HACK with the static QAction for minimalViewAction, and fix a few minimalVIew issues.

 * add the possibility to go slower and faster (wasn't finished)
 * some cleaning and factorization

Change wording to be more accurate.

And maybe make JB happy.
Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>

(and strlen("item-current") == 12)