Use the manifest meta-content system to add suitable tags for Qt WebKit
examples.

This enables users to filter for them in Qt Creator Welcome mode by
typing 'webkit'.

Change-Id: Ie9b5f6ff9f455dd3d8f7c64e5f73e30ff021f208
Reviewed-by: Kai Koehne <kai.koehne@digia.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>

Add an 'isHighlighted' attribute to selected examples to bring them
to the top on Qt Creator Welcome mode/Examples tab.

Task-number: QTBUG-29354
Change-Id: I6c72c11f1f8359d889649ff5b7f91e5c4de583bd
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>

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>