Merge pull request #516 from est77/master

DisneyBRDF updates #1
Fixed bug
Importing and exporting disney materials

They are exported using individual files. Currently the settings.xsd
format is used and some extra parameters are added to save the name
and the model of the materials.

Contains also a refactoring of show_message_box() from expressioneditorwindow
which now is required in multiple places. It has been moved to tools.cpp
and renamed to show_warning_message_box().

Signed-off-by: Marius Avram <marius.avram1309@gmail.com>
Merge pull request #517 from marius-avram/import_export_materials

Importing and exporting disney materials
Small disney material editor improvements and fixes.
Added expression examples browser

The expression browser can be shown/hidden by clicking
the "Examples" button.

Signed-off-by: Marius Avram <marius.avram1309@gmail.com>
Combine 2 #ifdefs in one.
Merge pull request #519 from marius-avram/examples_browser

Added expression examples browser
Merge pull request #518 from est77/est

Small disney material editor improvements and fixes.
Made refraction tests more robust in microfacet BTDF, fixed bug.