fix, test, and document more Inkscape::URI methods
Completes tests and documentation for:
- from_native_filename
- getFragment
- getOpaque
- getPath
- getQuery
- getScheme
- isNetPath
- isOpaque
- isRelative
- isRelativePath
- isAbsolutePath
Marks as deprecated:
- fromUtf8
- getFullPath
Fixes:
- isNetPath
- isRelativePath
- isAbsolutePath
- isOpaque
- getOpaque
Note: some of these methods are unused and probably irrelevant for Inkscape (e.g. "isNetPath" or "isAbsolutePath").