silx.gui.plot.PlotWidget: do not reset zoom when changing axes scales

[t20100]

This PR replace the previous unconditional resetZoom when switching plot axes scales to one that tries to preserve axis limits as much as possible:

• When switching from "log" to "linear" scale, limits are not updated
• When switching from "linear" to "log" scale:
  • Only the axis that has a scale change is updated.
  • If limits are positive, they are kept
  • If min limit is negative, the data range is used for setting it and the max value is kept if positive and having data below it.

closes #2825