[effects] Cleanup screenshot effect

Summary:
This change cleans up the screenshot effect a little bit.

• better check whether a screenshot is already being taken
• proper DBus error messages
• less duplication of error message strings
• don't keep the QDBusConnection around

Reviewers: KWin, Plasma

Subscribers: plasma-devel, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D3493