Outstanding issues
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(see also todo.txt)

- wxTextCtrl doesn't work as a stream buffer in Linux/gcc
- No wxSpinButton
- Need a way of specifying default settings for colour/font/etc.
- More optimisation could be done to reduce X server traffic, etc.
- Should scrollbars take on the background colour? Not right for
  e.g. wxScrolledWindows, so maybe have wxSystemSettings value for
  scrollbar colour, and/or ability to set scrollbar colour independently.