windeployqt: Remove deleted QML directories from Json output.

Rewrite JsonOutput to use a QPair<> of strings and add function to remove
all entries of a target directory.

Task-number: QTBUG-39626
Change-Id: Ie4602deb45d60fdddab27f264ecf34b72f839254
Reviewed-by: Andrew Knight <andrew.knight@digia.com>