Workaround problems with Qt::QueuedConnection

Summary:
For unknown reasons our signals with Qt::QueuedConnection are not
delivered with Qt 5.6 if the context object is this (KSldApp instance).

If we use a different context object (e.g. the sender) it works.

The lack of signals not working triggered quite a few bugs, like

• grace time not working
• global shortcuts not working

BUG: 361008
BUG: 361007

Reviewers: Plasma

Subscribers: plasma-devel

Projects: Plasma

Differential Revision: https://phabricator.kde.org/D1314