Change default behavior to remembering the selected region until Spectacle is closed

Summary:
Change the default to remembering the selected region until the next restart while still preserving the old behavior if a user had previously selected "remember region" which worked across restarts.

FEATURE: 391299
FIXED-IN: 19.04.0

Reviewers: #spectacle, #vdg, ngraham

Reviewed By: #spectacle, #vdg, ngraham

Subscribers: #spectacle

Tags: #spectacle

Differential Revision: https://phabricator.kde.org/D19130