Fixed min and max label values not being swapped when using wxSL_INVERSE in a horizon...

Fixed min and max label values not being swapped when using wxSL_INVERSE in a horizontal MSW slider.

Changed wxSlider::GetLabelsSize to let the widthMin and widthMax values contain the width of what the minimum (always on the left/top) and maximum (always on the right/bottom) _label_ will contain. This is not the same as the minimum and maximum _value_ in case of using wxSL_INVERSE.

Also don't swap the position of the min/max labels in a horizontal slider with wxSL_INVERSE (just how the labels already stay in place in a vertical slider, regardless of wxSL_INVERSE usage), nor their width.

Regression since r65941.

Fixes #13416.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@70048 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775