Add support for wxSL_MIN_MAX_LABELS and wxSL_VALUE_LABEL to XRC

Recognize these flags in wxSlider XRC handler.

Closes #16949.

Closes https://github.com/wxWidgets/wxWidgets/pull/253

src/common/slidercmn.cpp

@@ -66,6 +66,8 @@ wxBEGIN_FLAGS( wxSliderStyle )
     wxFLAGS_MEMBER(wxSL_HORIZONTAL)
     wxFLAGS_MEMBER(wxSL_VERTICAL)
     wxFLAGS_MEMBER(wxSL_AUTOTICKS)
+    wxFLAGS_MEMBER(wxSL_MIN_MAX_LABELS)
+    wxFLAGS_MEMBER(wxSL_VALUE_LABEL)
     wxFLAGS_MEMBER(wxSL_LABELS)
     wxFLAGS_MEMBER(wxSL_LEFT)
     wxFLAGS_MEMBER(wxSL_TOP)