[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