FrameSvgItem: add "mask" property
Summary:
This property allows e.g. the Plasma shell to query the shape a panel has
when defining the area for BlurBehind.
Reviewers: Plasma, mart
Reviewed By: Plasma, mart
Subscribers: kde-frameworks-devel
Tags: Frameworks
Differential Revision: https://phabricator.kde.org/D20202