Flint: Make it easy to switch between different kind of scans

Closes #3667 (closed) Closes #3112 (closed)

This PR rework part of the logic of the update of a curve plot from a new scan to provide a better selection especially when scientists have to change the kind of scans.

This is mostly handle by an additional lock on the curve property which allow to lock the axis or the counter selection independently.

Plus few other fixes of problem discovered during the tests.

Previously

The following sequence of scans was displaying an unusable result, forcing people to reselect the stuffs in the UI.

ascan(robx, 0, 10, 10, 0.1, diode1, diode2)  # this displays robx/robx which is POINTLESS(1)
# select the diode2 in the UI                # this displays diode2/robx
ascan(roby, 0, 10, 10, 0.1, diode1, diode2)  # this displays NOTHING(2) anymore

Now

It make it easy to select a set of counters and to switch scans between

ascan robx
ascan roby
loopscan 

And for some other use cases the x-axis can be enforced in the UI side.