Improve qmlimportscanner.

Fix several qmlimportscanner defects:

- Expand qml file search to include subdirectories.
This applies to search paths specified with "-rootPath"

- Add "-qmlFiles" for specifying the file list directly

- Parse files with UTF8 BOMs correctly

- Don't set properties where the value is the empty string

Change-Id: I81a538d73487b49fb635c18ef4fe479748ba0bfc
Reviewed-by: Morten Johan Sørvig <morten.sorvig@digia.com>