Improvements in usability for selecting shaders in 3Delight/blender, using new blender python API features from 2.58.

Now all shaders from your shader path are scanned and put into a list to select from, which updates dynamically when the contents of a folder in the shader path changes. There's also no more requirement to click on 'initialise shader parameters'.

There's quite a lot of shaders included by default with 3Delight, making for a pretty big menu. In future releases I plan to write and include a few decent general purpose shaders with the exporter itself as a default set, and disable scanning using the 3Delight example shaders by default.

To keep the UI responsive, the scanning is done in a background thread and cached - it's only regenerated when a folder in the shader path is modified, and no more frequent than every 2 seconds.