Add "qtwebengine", "webengine" tags to all Qt WebEngine examples

The new MarkDown Editor example shows up in Qt Creator, but there's no easy way to see it's actually a webengine example. Current tags are just
"markdown" "web" "engine" "editor"

IIRC there's a possibility to add tags to every example. I think all webengine examples shold have the tag "qtwebengine", and an optional "qtwidget" or "qtquick" tag (depending on which integration they use).