further support of XDXF rev 32 (thx to @Abs62)
0. <abbreviations> are now processed depending on format revision
1. <mrkd> tag supported now;
2. <tr> for old visual XDXF files fixed;
3. <def> `cmt` attribute is supported now;
4. <kref> `kcmt` attribute is supported now;
5. some small coding conventions