[QScintilla] Problems when pasting rectangular selections

[Baz Walter]

If the clipboard has support for selection mode, it should be possible 
to paste the current selection using a middle-button click.

This currently works correctly for ordinary selections, but not for 
rectangular selections. I can only paste rectangular selections if the 
selection is cleared first - otherwise, what appears to happen is that 
the selection just overwrites itself.

P.S.

Whilst experimenting with this, I also find another, more minor bug.

Looking at QsciScintillaBase::fromMimeData, it appears that a trailing 
"\n\0" is used to identify rectangular text. However, this won't work 
for sources using Mac line-endings, because those will have a trailing 
"\r\0".