swapdmt: Compatibility with wxWidgets 3.0

For compatibility with wxWidgets 3.0, not only the imports need to be
changed, but also the interface for wx.CallAfter() seems to have
changed. Additionally, a menu index of 0 is invalid on OS X, so start
with the first menu entry at position 1.
Merge pull request #4 from raimue/swapdmt-wx30

swapdmt: Compatibility with wxWidgets 3.0