olevba: improve parsing of XLM macros

[decalage2]

I need to improve the parsing of XLM macros, probably something like this:

• if format = OLE:
  • if XLMMacroDeobfuscator is installed, use it.
  • if not, or if XLMMacroDeobfuscator triggered an error or found nothing, use plugin_biff.
  • if plugin_biff triggered an error, report it and check if there are XLM macros or not.
• if format = OpenXML (not XLSB):
  • if XLMMacroDeobfuscator is installed, use it.
  • if not, or if XLMMacroDeobfuscator triggered an error or found nothing, use a simple XML parser to extract macrosheets.
• if format = XLSB:
  • if XLMMacroDeobfuscator is installed, use it.
  • if not, or if XLMMacroDeobfuscator triggered an error, display warning that XLM macros cannot be detected.
• if forrmat = SLK:
  • use internal parser

Related issues: #728, #742, #741