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