Add option to auto-accept box selection on mouse release

Summary:
FEATURE: 404829
Adds the option to have spectacle accept the selected region as soon
as the mouse button is released, similar to how other screenshot tools
allow it (eg. scrot -s).

Test Plan:
To enable the option go to
Configure -> General -> Rectangular Region -> Accept region on mouse release
then set the Capture mode to Rectangular Region and take a new screenshot.

Behaviour should be the following:

-When selecting a new region, without a previously saved region available,
 click->drag->release should immediately capture the selected region.
-When a previously saved region is shown, the saved region can be
 manipulated further and saving this region requires manual confirmation
 (Enter/Double LMB).
-Clicking and dragging outside of the previously saved region
 captures the newly selected region immediately.

Reviewers: VDG, Spectacle, ngraham

Reviewed By: VDG, Spectacle, ngraham

Subscribers: ngraham, Spectacle

Tags: Spectacle

Differential Revision: https://phabricator.kde.org/D19525