Swap GUD fringe/margin mouse-2 and mouse-3 bindings

[Kim F. Storm]

IMO, we should swap "toggle enable/disable breakpoint" (on mouse-3)
and "run until" (on mouse-2).  

In normal bindings, mouse-3 is "extend region until click position",
so it resembles "execute until click position".

Likewise, mouse-2 normally is "yank at point", which resembles "toggle
at point".

I guess that "run until" is used more frequently that "enable/disable
breakpoint", so let's make it easier to use "run until" for the poor
soles with a two-button mouse (e.g. most laptop users).


--
Kim F. Storm <address@hidden> http://www.cua.dk