Fix showing slider label in wxMSW when wxSL_MIN_MAX_LABELS was not used.

Fix showing slider label in wxMSW when wxSL_MIN_MAX_LABELS was not used.

wxSlider with wxSL_LABELS style but without wxSL_MIN_MAX_LABELS didn't show
the current value because the width of the value label was 0.

Fix this by always computing the widths of min and max labels and using the
longest of them for the value label, even if we don't actually show them.

Closes #13291.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@68230 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775