Change the logic of open line commands so that if a selection lies
on the end of line character of the line from which we open a new
line, that selection does not move.
If we have two clients, A and B, with B's cursor on the eol character
of line L, and A hits `o` while on line L, B's cursor should stay
on the same (logical) line. Previous behaviour would make B's cursor
jump on the newly inserted line.
Various places in Kakoune code used to modify selections so that
cursors would not lie on an end of line. Remove those to increase
Kakoune's consistency and simplicity.
Now that end of lines are highlighted separately, they should not
be handled specially in most commands.
Avoiding end of line is a behaviour we avoid (pun intended) more and more
in Kakoune source code, now that end of lines are regularly selected, it
makes no sense to just to next line when last modification lies on an EOL
(and it probably did not make much sense back when that code was written).
