Stop hardcoding focus border size in collapsible pane header

Use the theme- and DPI-dependent size of the border instead.

See https://github.com/wxWidgets/wxWidgets/pull/1580

src/generic/collheaderctrlg.cpp

@@ -30,6 +30,10 @@
 
 #include "wx/renderer.h"
 
+#ifdef __WXMSW__
+    #include "wx/msw/private.h"
+#endif // __WXMSW__
+
 // if we have another implementation of this class we should extract
 // the lines below to a common file
 
@@ -95,7 +99,8 @@ wxSize wxGenericCollapsibleHeaderCtrl::DoGetBestClientSize() const
         size.y = textSize.y;
 
 #ifdef __WXMSW__
-    size.IncBy(1);
+    size.IncBy(wxGetSystemMetrics(SM_CXFOCUSBORDER, this),
+               wxGetSystemMetrics(SM_CYFOCUSBORDER, this));
 #endif // __WXMSW__
 
     return size;