Make it more clear that we do what we do in DoGetBestClientSize()
because of what our OnPaint() does.
Incidentally fix off by 1 (or 2 under non-MSW platforms) mismatch
between the 2 methods: the gap between the button and the text is
actually just 2 DIPs, not 4, but we need to add another 1 DIP for the
focus rectangle under MSW.
This simple class has a collapsed state which is indicated by a small button and a label. It is now used in the generic implementation of wxCollapsiblePane.
It could be used in more complex layouts than wxCollapsiblePane and is therefore available and documented to the user.
