Fix WXGetManuallySetWidth() for columns using wxCOL_WIDTH_DEFAULT

Columns without any explicitly specified width still shouldn't be shrunk
down to 0 size by UpdateColumnSizes(), so handle them as if they were
created using wxDVC_DEFAULT_WIDTH instead -- which is what their actual
width is/would be.

This is a better fix than the one in 0c90ea40c3 (Don't auto-resize
wxDataViewCtrl columns below their initial size, 2019-10-03) and that
commit can be reverted now, as will be done soon.

See #18343.

include/wx/generic/dataview.h

@@ -154,7 +154,7 @@ public:
     // user interactively.
     void WXOnResize(int width);
 
-    int WXGetManuallySetWidth() const { return m_manuallySetWidth; }
+    int WXGetManuallySetWidth() const;
 
 private:
     // common part of all ctors

src/generic/datavgen.cpp

@@ -217,6 +217,15 @@ void wxDataViewColumn::WXOnResize(int width)
     m_owner->OnColumnResized();
 }
 
+int wxDataViewColumn::WXGetManuallySetWidth() const
+{
+    // Note that we need to return valid value even if no width was initially
+    // specified, as otherwise the last column created without any explicit
+    // width could be reduced to nothing by UpdateColumnSizes() when the
+    // control is shrunk.
+    return DoGetEffectiveWidth(m_manuallySetWidth);
+}
+
 void wxDataViewColumn::UpdateDisplay()
 {
     if (m_owner)