Fix for #948 (bug where not all text was copied to the clipboard when copying from...

Fix for #948 (bug where not all text was copied to the clipboard when copying from flat review for a display model due to NULL characters). Specific changes:
*Implement a new clipboardText property on the base TextInfo class, which returns the text property, processed by convertCrlf.
*TextInfo.copyToClipboard: pass the clipboardText property to api.copyToClip rather than convertCrlf(self.text).
*Base virtualBuffer: rather than implementing a copyToClipboard method, instead implement a clipboardText property. It still inserts linefeeds in between block fields etc, but just returns the text, rather than copying it to the clipboard now of course.
*DisplayModelTextInfo: implement a clipboardText property, which fetches its super clipboardText property, but converts NULLs to spaces before returning.