Add date-related info insets

Add date-related info insets

With this commit, info insets leave the dark backstage room of an opaque
and quite hidden dev-only feature and come frontstage.

In the UI, they present themselves as "Fields" since this is what people
know from word processors. Other user-related fields that could be
implemented next: time, user name (I plan to do that for 2.4).

Since this supersedes date-insert, I removed Insert > Date from
the menu and propose to ditch date-insert and the corresponding rc.

The lyx2lyx reversion routine has lots of room for improvement and
attractive tasks for pythons (file timestamp, switch of localization).
Please feel invited!

This is a file format change.