more complete support for workspace styles

follow up to 068e604 and some work from @volaya extracted from the
suite-qgis-plugin 'fork':

- simplify Catalog.get_style_by_url
- make Catalog.create_style work for workspace styles
- support for layer setting workspace styles
- add fully qualified name support for styles (workspace:name)
- test coverage
Merge pull request #57 from ischneider/style-namespaces

more complete support for workspace styles