Add search path for shader files.

Helper: initShaderState(...) gets additional search path parameter.
ImportContext: Store an array of shaderSearchPaths.
Remove ShaderFactory (approach seems too complex)
Support default shaders with names.
SceneManagement: Use FileLocator for finding files.

ImporterTools: remove unused checkRelativePaths(...)
Use FileLocator for locating files where possible.

CoreNodeExporter: Don't change meshs' filenames when exporting.
CoreNodeImporter/CoreStateImporter/MeshImportHandler: Use FileLocator to locate files.