[Cin] Today's GIT checkin includes fixes + Ctrl-s for save session

[Andrea paz]

Thank you for the many changes.

- By now I am used to "s" and "z" and I think I will continue to use
these, but it is a good thing to meet the needs of new users.

- Excellent "drag speed highlights", really useful. The bug reported
by Igor seems fixed.

- F_minterpolate works but it is very heavy, also because it only
works single-thread. Strange because normally ffmpeg plugins can use
multi-threads.

As for native "Interpolate Video" plugin; it seems to me that it works
well in fast motion while it doesn't work well in slow motion, giving
me these 2 errors to the terminal:

MotionHVScan::pixel_search 535 range fail range1=0.000000
MotionHVScan::pixel_search 548 range fail range2=0.000000

I think, if possible, it would be better to fix this native plugin
than to use the ffmpeg plugin.