Summary:
The effect exports itself to DBus as object "/ColorPicker" and provides
an own interface "org.kde.kwin.ColorPicker".

It has one exported method to DBus "pick" which returns a QColor. When
invoked an interactive position picking selection is started. If it ends
the effect reads the color value at the picked position from the OpenGL
color buffer.

This implements T4568.

Reviewers: #kwin, #plasma_on_wayland, broulik

Subscribers: plasma-devel, kwin

Tags: #plasma_on_wayland, #kwin

Differential Revision: https://phabricator.kde.org/D3480