By default when a plot is added to the plot model, the
importance value is set to "0" (Minor). However when the new
plot is selected, the importance slider remains set to the value
of the previously selected plot.
Steps to Reproduce:
1. Click add plot (plus sign) button.
2. Select this "New plot".
3. Select "Basic infos" tab and change Plot name to "My Plot".
4. Move Importance slider all the way to the right (Main).
5. Click add plot (plus sign) button.
6. Select this "New plot".
Note that the "New plot" is shown in the plot list pane under
"Minor", but the Importance slider is still all the way to the
right (Main).
This enhancement ensures that the corresponding plot UI importance
slider is also set to the proper plot importance.
