Make more signals non-const

Summary:
Not sure exactly what is wrong with const signals, besides
QMetaObject::activate(...) wanting a non-const this argument.

Collected signals are from a recent clazy run, so changing these will at least
result in less noisy future clazy run ;)
And perhaps consistency in general signal non-constness.

The Clazy const-signal-or-slot check docs only claims
"For signals, it's just pointless to mark them as const."
without giving any reasoning.

So dumping patch as RFC, to be either moved to /dev/null or applied.

Reviewers: KDevelop, apol, kfunk

Reviewed By: KDevelop, apol, kfunk

Subscribers: apol, kdevelop-devel

Differential Revision: https://phabricator.kde.org/D8959