Support multiple event types in X11EventFilter

Summary:
So far the X11EventFilter only supported one event type. But most
usecases for an X11EventFilter require to support multiple event types.
E.g. a filter listens for both key press and key release.

This change extends the internal X11EventFilter to support multiple
types and Workspace::workspaceEvent makes use of that.

Reviewers: #kwin, #plasma

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D3361