Fix horizontal scrollbar handling for wxCheckListBox in wxMSW

The scrollbar wasn't shown when the control became only slightly more
narrow than its contents, resulting in truncation of the rightmost part
of the strings shown in it.

Fix this by accounting for the check box explicitly in wxListBox
SetHorizontalExtent() method using the new MSWGetFullItemSize() helper
which is also used to avoid code duplication between wxCheckListBox
MSWOnMeasure() and DoGetBestClientSize() methods.

Closes #18377.

include/wx/msw/checklst.h

@@ -65,6 +65,8 @@ public:
     virtual bool MSWOnMeasure(WXMEASUREITEMSTRUCT *item) wxOVERRIDE;
 
 protected:
+    virtual wxSize MSWGetFullItemSize(int w, int h) const wxOVERRIDE;
+
     // pressing space or clicking the check box toggles the item
     void OnKeyDown(wxKeyEvent& event);
     void OnLeftClick(wxMouseEvent& event);