New Layer from visible

Summary:
its a common function under Layer->New menu which is in gimp and ps.
It create /insert layer on top from visible pixels

Test Plan: u can create several layers with some painting and then test this function which create new layer with all paintings together

Reviewers: rempt, dkazakov

Reviewed By: dkazakov

Differential Revision: https://phabricator.kde.org/D2428