Collapsible pane notes

[ABX]

Hi (Francesco, Vadim)!

I looked into wxWinCE appearance of this new feature and it doesn't look nice
in about example of 'dialogs' sample. I tried to look into code myself byt I
don't want to interfere if you have some work in progress around it. So here
are some my notes:

1. wxCP_MARGIN - this is hardcoded. For consistency I believe this should be
same value as in sizer created by wxDialog::CreateButtonSizer.

2. Static line - I suppose this line has same rules and borders as static line
created by wxDialog::CreateButtonSizer() ie. probably it should not appear on
wxMac to follow HIG guidelines (probably Stefan will opinion this better).
Perhaps some of the code could be shared here.

3. Not sure if that sounds strange but... could it be that now with RTL
support in GUI, directions in wxGenericCollapsiblePane::GetBtnLabel() could be
sometimes reversed?

4. How does wxGenericCollapsiblePane::OnStateChange() is supposed to work if
wxTLW is maximized? This is the case of wxWinCE and things look strange here.

HTH,

ABX