QtQml: document QQmlExtensionPlugin::baseUrl

It was added here https://codereview.qt-project.org/#change,47532
without documentation.  Also updated \since because for now qdoc
seems to interpret \since 5.0 as QtQml 5.0.

Task-number: QTBUG-31928
Change-Id: I652e9471f1ad8e83b355bffc8498a6565dc9aa2b
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Reviewed-by: Martin Smith <martin.smith@digia.com>
FileDialog.folder property should also be a QUrl, for consistency

In QtQuick we never use plain file paths, because URL is more general.
Also use const references for string and URL setters, and fixed the
dependency between the FolderListModel's folder, the field for editing it,
and the folder property of the AbstractFileDialog.

Change-Id: I6e965b80b73d4eb2473712a4f4d4f816b768d802
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>