Doc: Support for meta-content in manifest XML files

This change makes qdoc support additional attributes and tags written
to example/demo manifest files. The goal is to enable highlighting of
selected items, as well as having additional content to make searching
for specific categories work better in Qt Creator welcome screen.

This meta-content is stored in manifest-meta.qdocconf, which is loaded
globally for all modules.

Tag handling is also changed to use a QSet to eliminate possible
duplicate tags.

Task-number: QTBUG-29354
Change-Id: I2c4b2dff6229172efbecc2bfc1c269017edc4d56
Reviewed-by: Martin Smith <martin.smith@digia.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>