Show elements' id on mouse hover

[bertsky]

PageViewer shows a mouse-over with some extra info for different segment types:

• segment @id
• segment @orientation (if applicable)
• segment @type (if applicable)
• segment @{primary,secondary}{Script,Language} (if applicable)
• segment @readingDirection (if applicable)
• segment @textLineOrder (if applicable)
• segment @custom (if any)
• segment Coords/@conf (if any)

Not all of these are equally important, but being able to identify segments by name this way would already be extremely useful for LAREX as well IMHO.

(If this interferes with normal mouse control for navigation and editing, perhaps some keyboard modifier could be used to restrict this feature? For example, the Alt key still seems to be available...)