cursor positioning is wrong when sending commands

[alex heneveld]

the GotoAction? will move the cursor to near the end of the previous command. that's annoying, if i'm using the GotoAction? then presumably i'm happy with where the cursor is (typically i'm writing a line of a proof, and i want PG to prove everything up to it.)

next and undo also move the cursor position -- which sometimes i like and sometimes i don't. not a bug in itself but worth considering what desired behaviour should be whilst fixing the bugs above and below.

there are also some obvious off-by-one errors when setting cursor position, it ends up before the last processed char, rather than after it.

and there are some curious races in setting the cursor position -- sometimes it gets moved and sometimes it doesnt.