Implement the editing of fields

[dirkroorda]

The edit buttons are already there, subject to authorisation.

Now display an editable version of a field, with save/cancel/exit buttons.

Upon save, report succes/failure in the message bar. Upon exit, change to the readonly version Upon cancel, change to the orignal, readonly version.

Upon leaving the field, save it.

Relation with the issue on preventing edit conflict.

[dirkroorda]

We require explicit saving.

Press cancel once to return to the last saved value, still in edit mode. Press OK to leave edit mode.

[dirkroorda]

Done