[ksgrd] Drop a QWidget dependency

Summary:
The broadcaster just needs to receive a QEvent getting propogated
upwards, it doesn't need to be a QWidget.

This makes the client API considerably lighter.

.so is bumped as it is technically an ABI break.

Test Plan:
Ksysguard still works
lddtree shows everything being much lighter

Reviewers: Plasma

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D26870