Consistently handle invalid and deprecated tags in setProperties()

This commit reverts the use of strip() in setProperties() because the
latter function should not change the file before save() is called.
Instead, the following policy is now consistently applied for file formats
with multiple tag types:
- the recommended tag type is created, if it does not exist
- deprecated tags are updated, if they exist, but not created
- illegal tag types are ignored by setProperties(), but used in properties()
  if no others exist.

The only tag types considered "illegal" so far are APEv2 in MPEG and ID3 in FLAC.