Added a label to guide users to line style settings in the Render Layers context
(in line with the similar labels found in the Cloth and Soft Body modifier panels),
so that Freestyle options per render layer can be more easily found.
* Do not share poll with other files, create own poll classes.
* Avoid some splits() and use rows instead (less code).
* Remove some commented C code.
* layout = self.layout declarations come before variable declarations.
Suggested by Brecht Van Lommel and Campbell Barton through code review comments.
Previously style modules were external Python script files whose absolute paths
were kept in .blend files. Now style modules are stored in .blend files as text
datablocks.
Style modules are configured in three steps:
1. Open an external style module file (or create a new text datablock) in the
Text Editor in Blender.
2. Add a style module to the list of style modules (by pressing the "Add" button)
in the Render Layer properties window.
3. Click the name entry and select the style module from the drop-down menu.
