Add 'markdowneditor' example

This example shows the use of QWebEngineView in a hybrid application,
and how one can leverage JavaScript libraries to provide functionality
with minimal effort.

QWebEngineView is used to preview a MarkDown document. The text is
exposed to the view through QWebChannel. An off-the-self js library
converts it to HTML.

Change-Id: I24c38106da3ec18975c71c16f7f7a58e93142f9e
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>