Summary:
I'm trying to implement the idea of @davidedmundson from this https://phabricator.kde.org/T10172 with my limited knowledge of C++/Qt/KDE/programming. This is the start of a new class so it could lack many things but it's usable.

- It allows clients to set the available screen rect/region from outside through dbus.
- The final available screen rect/region would be the intersection of all set values.
- When a client dies, its values would be removed automatically.

Test Plan:
+ qdbus-qt5 org.kde.plasmashell /StrutManager test org.kde.lattedock LVDS-1 0 24 1366 768
+ After killing latte-dock, the values are got only from plasmashellCorona.

{F7820969}

Reviewers: #plasma, mvourlakos, davidedmundson, mart, apol

Reviewed By: #plasma, davidedmundson

Subscribers: davidre, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D25807