Patch: use std::string

Hi,

Another patch for the impexp code. In IE_MimeConfidence and
IE_SuffixConfidence replace gchar * by std::string. Why? Resource
tracking is made easier. There are case where a static string cannot
just be used.

Attached are the patches. Broad but not really intrusive. One for abi,
one for plugins.

Feedback? (I was about to commit it but earlier got asked for a patch
when I proposed the change)

Hub