Doc: Set priority for highlighted examples

QDoc allows tagging examples with extra 'meta' attributes - we use it e.g. to tag certain examples to be highlighted in Qt Creator. There are multiple highlighted examples now, and they are presented in alphabetical order by the example title.

Add an additional attribute to each highlighted example, e.g. 'priority', to control the order in which the examples are listed. Qt Creator needs to be made aware of this attribute, too.