lang: Add editable property to QWebView

SCWebView had this feature, but QWebView never has. It's not needed for help files, but this provides an easy way to edit examples in rich text files without having to write javascript.

Signed-off-by: Scott Wilson <i@scottwilson.ca>
Merge pull request #969 from supercollider/AddEditableToWebView

lang: Add editable property to QWebView