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.

{F6657239}

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