fix: restore Alt+Mousemove cross-section update on images

Regression introduced in a522dcb ("Cross section plots: fixed historic
flaw"), first released in v2.1.0.

Closes #68

Author: PierreRaybaut

plotpy/panels/csection/csplot.py

@@ -436,7 +436,7 @@ def connect_plot(self, plot: BasePlot) -> None:
         )
 
 
-class XCrossSectionPlot(HorizontalCrossSectionPlot, XYCrossSectionMixin):
+class XCrossSectionPlot(XYCrossSectionMixin, HorizontalCrossSectionPlot):
     """X-axis cross section plot"""
 
     _HEIGHT = 130
@@ -450,7 +450,7 @@ def create_cross_section_item(self) -> XCrossSectionItem:
         return XCrossSectionItem(self.param)
 
 
-class YCrossSectionPlot(VerticalCrossSectionPlot, XYCrossSectionMixin):
+class YCrossSectionPlot(XYCrossSectionMixin, VerticalCrossSectionPlot):
     """Y-axis cross section plot"""
 
     _WIDTH = 140