This adds an action that can be triggered on any non referencing layer(so not file or clone layers).

This action will request a file name, and then save the layer(if a group layer, with everything in it)
into a seperate file. It will then replace the layer in the old with with a file layer referencing the
new file.

The idea behind this one is that if people's layer stacks become too complicated,
they can save out a section of it to a file layer, reducing the complexity of the document.

CCMAIL:Kimageshop@kde.org
Differential Revision: https://phabricator.kde.org/D8360