[effects] Add interactive window selection mode to ScreenshotEffect

Summary:
EffectsHandler gains a new method to startInteractiveWindowSelection
which just delegates to the one in Platform. That way a window can be
selected and returned to an Effect.

The screenshot effect makes use of this new functionality and provides
an interactive window screenshot mode which saves to a temporary file.
Note that this is not yet the variant intended for use in spectacle.

Test Plan: Took a screenshot on Wayland

Reviewers: #kwin, #plasma_on_wayland

Subscribers: plasma-devel, kwin

Tags: #plasma_on_wayland, #kwin

Differential Revision: https://phabricator.kde.org/D3367