Improve texl2yx refstyle support

Improve texl2yx refstyle support

- Convert prettyref to the autopackage mechanism
- Do not load refstyle automatically if some refstyle preamble code of LyX
  is found, since LyX will only load the package if an actual reference
  command is used. This is needed for mixed refstyle/prettyref documents.
- Only recognize refstyle commands if refstyle was detected in the preamble
- Only recognize prettyref commands if prettyref was detected in the preamble
- Add a mixed refstyle/prettyref test case