Add interactive position selection to screenshot screen under cursor

Summary:
A second interactive selection mode gets added to select a position on
the screen. This is handled by the same input event filter as for the
window selection. Just that instead of returning a window, it returns a
QPoint.

This allows to pick a point on the screen which we need to screenshot
the screen under the mouse cursor and in future for color picking.

The screenshot effect provides two new dbus methods to (interactively)
select a screen or fullscreen. This allows spectacle to screenshot the
(full) screen with still having the user in control.

Reviewers: KWin, Plasma on Wayland, bgupta

Subscribers: plasma-devel, kwin

Tags: Plasma on Wayland, KWin

Differential Revision: https://phabricator.kde.org/D3475