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