wxWidgets 2.8 defines file dialog enums as wxFD_... while 2.6 used wx_...
This patch detects whether or not the wxFD_... enums are available and
uses them; otherwise the old enums are used.
On Linux wxWidgets 2.8.0 and newer uses the GTK combo boxes which suck. A lot.
Therefore mmg uses wxBitmapComboBoxes for wxWidgets >= 2.8.0 on Linux and normal
wxComboBoxes in all other cases. wxBitmapComboBoxes are still drawn by wxWidgets
itself (just like wxComboBoxes before 2.8.0) and offer much better functionality.
Fix for bug 339.