This introduces the concept of clipboard history groups. A group is created
for each high level clipboard operation. Cut, copy, etc. Single cursor
operations will create a group with just one entry. Multi-cursor operations
on the other hand will create groups with multiple clipboard history entries.
This makes for very powerful clipboard history integration with multi-cursor
support.
This commit also adds the ability to apply integer parmeters to the paste
command to select a clipboard group to paste.
Also, pasting from the system clipboard will detect if the system clipboard is
equivalent to the top most clipboard group, and if so use the group instead.
This allows much better multi-cursor support when using the system copy & paste
commands.
