Doc: Use \qml for code snippets where possible

QDoc's QML parser works OK for code snippets that include a full
type declaration; use \qml for these snippets to get autolinking
to QML types working.

Use \badcode for codeblocks that are not code or have nothing to
link to.

Change-Id: Iffd2f356bf0b3efc854a3755873e66f8c9478cac
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@theqtcompany.com>