olevba: detect and extract Excel 4 Macros (XLM/XLF) in XLSM

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See #389 for details

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Sample: https://twitter.com/DissectMalware/status/1250409114435551238

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@DissectMalware developed a parser for XLSM to extract XLM macros (Apache license):
https://twitter.com/DissectMalware/status/1250495410860371970
https://github.com/DissectMalware/XLMMacroDeobfuscator/blob/macro_emulator/xlsm_wrapper.py

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TODO: check changes from PR #569, improve with an XML parser and/or use XLMMacroDeobfuscator