[lehni]

Closes #2462, closes #2463, closes #2464

• Linked text (Hosted: Moving linked text to another host keeps sharing its story #2462): a text item of a linked story that moves into a project with another host brings a copy of the whole story along, as its own unlinked story, as clone() does for copies of text. The story flows on through the items that stay, and the old project logs their change.
• Definition copies (Hosted: Copies of definitions don't follow their originals #2463): projects no longer keep the copies of other hosts' definitions. Each insertion, layer insertion or copy into a project with another host brings its own copies along, shared by the items it brings, so later changes or deletions can't leave stale or empty copies behind. Moving a definition's items in two separate insertions now creates two copies.
• Record flags (Hosted: Name the flags of host records #2464): the flags of the host's records are named constants inside Item.Hosted.js.
• Project#takeChanges(): items that moved to another project since they were logged are left out, as that project lists them. Before, an item changed in one project and then moved to another stayed in the first one's log.

Tests: linked text moving to another host takes a copy of its story while the rest flows on, without being logged in the old project; a later insertion brings its own copies of definitions; and takeChanges() leaves out an item changed and then moved. All fail without the change. The test of shared copies now moves the items in one insertion.