desktoppackage: add "panelMask" property for Panel.qml
Summary:
Allows the panel code from plasmashell to know the shape of the panel, as
useful when defining the area for BlurBehind.
Reviewers: Plasma, mart
Reviewed By: Plasma, mart
Subscribers: plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D20203