Ticket #2956: Newer protocol for extended mouse clicks.

A followup extension, "SGR 1006" was invented by xterm, to overcome some
of the shortcomings of the previous one. It is becoming as widespread as
the previous one, and is likely to soon overtake it in popularity.

Note that most of the patch is just the removal of the huge complexity
introduced by the previous one. The previous extension didn't have a
unique prefix which made the whole parsing logic extremely complicated.
The new extension does have a unique prefix, so parsing becomes a piece
of cake. The code becomes much cleaner and much easier to maintain.

Signed-off-by: Slava Zanko <slavazanko@gmail.com>
Code refactoring: removed unneeded 'go to' label.

Signed-off-by: Slava Zanko <slavazanko@gmail.com>