common: Always let QVariant::fromValue() deduce the type

In some places, we would explicitly give the type to
QVariant::fromValue(), forgoing automatic type deduction. This is
almost always redundant, and in the cases where it is not, the
input type should be explicitly cast.

Additionally, the implementation of QVariant::fromValue() subtly changed
in Qt 5.14, leading to compile errors when giving a non-decayed type
(as we did in SignalProxy::attachSignal()).