The Scan entries in the editable Scan Lists (User default, Source,
Observe) may be edited either individually or in groups (Block Edit)
or in entirety (Global Edit).
The list of scans to be modified in a block edit are specified by the
user by selection the entries in the displayed ListBox.
Block edits are done by class
GUI.BlockEditBox
and global edits by
GUI.GlobalEditBox.
For either Block or Global edits, a scan is created in the Edit List
(the only Scan in this list) and the Scan editor is invoked on it.
When the scan editing is complete all explicitly set values in this
Scan are copied to the Scans selected to be edited by class
ApplyEdit
static member function update.
When the scans are updated they are replaced in the appropriate
GUI.ListBox.