Use a templated approach for InputRedirection::processFilters

Summary:
An improvement from the introduction of InputEventSpy. Instead of
specifying a std::function as argument, we let the compiler decide what
is the best argument.

Reviewers: #kwin, #plasma, mart

Reviewed By: mart

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D3877