[server] Reduce overhead of pointersForSurface

Summary:
In SeatInterface we need to get all PointerInterfaces related to a given
Surface (Client) and call a method on it. The implementation we had so
far went through all Pointers and put all PointerInterfaces into a new
temporary QVector. In most cases all we did then was iterating over the
returned vector.

Which means we created a temporary vector for nothing.

This change implements a kind of std::for_each with the constraints of
the previously used pointersForSurface which does the check that Surface
is not null and that the client matches. If a PointerInterface is found
for that, the passed in method is invoked on it.

Reviewers: #plasma_on_wayland

Subscribers: plasma-devel

Tags: #plasma_on_wayland

Differential Revision: https://phabricator.kde.org/D3295