Change the behaviour of the add layer button

Summary:
In the layers docker, if more than 1 layer is selected, then
the add layer button now creates a quick group for the selected
layers.

BUG: 385050
Reviewers: #krita, rempt
Reviewed By: #krita, rempt
Subscribers: alvinhochun, woltherav
Tags: #krita
Differential Revision: https://phabricator.kde.org/D11507

(cherry picked from commit 77d75e3d811add37b909683d61ad8e779f8524e1)