fragment descriptors in the DB

[olhsha]

Fragment descriptors for cached representations has a separate column in the table "cached representations". E.g. it may contain the position of a text fragment in the screen shoot.

Fragment descriptors of a targets is a parts of the link (starting from #) and does not have a separate column. Are we ok with it?

Also, the getting/posting annotation should be updated so that the link-attribute in "target" XML splits into two attributes: the link itself (before #) and the fragmemt-attribute. Now everything is in "link" and fragment descriptor is empty.