Let window size depend on the screenshot aspect ratio

Summary:
Now the window width size changes with the aspect ratio of the captured image.
Maximum width is 1000 pixels for this automatic resize.

Test Plan: {F5697651}

Reviewers: ngraham, #spectacle, rkflx

Reviewed By: ngraham, #spectacle, rkflx

Subscribers: rkflx, ngraham

Tags: #spectacle

Differential Revision: https://phabricator.kde.org/D10377