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.
That means we can now have highlighters active at global, buffer, and
window scope. The add-highlighter and remove-highlighter syntax changed
to take the parent path (scope/group/...) as a mandatory argument,
superseeding the previous -group switch.
Tabs now align to tab stops instead of always spanning 8 spaces when
show_whitespaces is added as a highlighter.
This fixes issue #1453.
A regression test is also provided.
