Reduce plasmashell frozen time

Summary:
Third part of https://phabricator.kde.org/D10627

According to the documentation at http://doc.qt.io/qt-5/qabstractnativeeventfilter.html

The type of event eventType is specific to the platform plugin chosen
at run-time, and can be used to cast message to the right type.

On X11, eventType is set to "xcb_generic_event_t", and the message can
be casted to a xcb_generic_event_t pointer.

The other eventType are "windows_generic_MSG" and "mac_generic_NSEvent".

No other eventType starts with an 'x'.

Reviewers: Frameworks, Plasma, mwolff, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: mwolff, plasma-devel, davidedmundson

Tags: Plasma, Frameworks

Differential Revision: https://phabricator.kde.org/D10670