Hide or disable some elements or attributes in the XML editor

[TejaGo]

In the schema designer, make it possible to hide or disable some XML elements and attributes so that editors can't change them in the XML editor. (Requested by Carole Tiberius.)

Even better, do this as a separate feature outside the schema designer. That way it will work even when the dictionary administrator has uploaded their own DTD (ie. when they're not using the schema designer).

[blahma]

This would be also useful for Lexical Computing. Often we need editors to be able to see values but not to be able to modify them. Note that flagging by clicking the dot next to a headword should not be affected even if the respective flag element has editing disabled (we want editors to flag by clicking the dot, but not to edit the flags manually, possibly introducing invalid values).

[michmech]

Xonomy already has support for this but Lexonomy isn't using it (yet). Same story as #26. The configuration of this should be under Configure » Entry editor.