Fixed styling issue on disney material layer folding

The label which remained once a disney material was folded changed
position to the left of its inner layout. This made the folded
and unfolded layers unconsistent. Now the issues is solved.

Signed-off-by: Marius Avram <marius.avram1309@gmail.com>
Save folded disney material layer state between sessions

When a project will be saved the folding state will also be saved
in the project file as a simple parameter with the true or false
value.

The next time the project will be opened the parameter will be used
to fold the layers the same way as they were before appleseed.studio
was closed.

Signed-off-by: Marius Avram <marius.avram1309@gmail.com>
Merge pull request #495 from marius-avram/folding_improvements

Folding improvements