Implement the same inline code identification as OmegaT's

[YvesS]

One important difference in today's Okapi filters compared to OmegaT's native ones is the way they name the inline codes.

Currently the plugin just uses the first letter of the XLIFF code. We should try to mimic the way codes are named by OmegaT.

See an example of the choice for the initial letter code here: https://sourceforge.net/p/omegat/svn/HEAD/tree/trunk/src/org/omegat/filters3/xml/xliff/XLIFFDialect.java

[Didier Briel]

The other important difference is that OmegaT resets the tag number at every paragraph.

Also, OmegaT respects the setting Remove leading and trailing tags in File Filters, but at least here the user can uncheck it to get the same configuration as Okapi's.

Didier