Fix the handling of hover and unhover events for all plot types

Description

This MR fixes two problems:

• the problem originally reported in #925 (closed) where only XYCurves were handled in CartesianPlotPrivate::hoverLeaveEvent().
• the problem with plots (xycurve, histogram, etc.) being not unhovered when moving the mouse from the plot to the axis and with both shapes overlapping, see for example this case with XYCurve touching the axis and the curve being not unhovered when moving the mouse down to the axis:

Furthermore, the custom handling of hover and unhover events in Histogram was removed, the implementation in the basis class has to be used instead.

Conformity

• Changelog entry
• Unit tests
• Update INSTALL
• Downport
• Fixes: #925 (closed)