`x` is often criticized as hard to predict due to its slightly complex
behaviour of selecting next line if the current one is fully selected.
Change `x` to use the previous `<a-x>` behaviour, and change `<a-x>` to
trim to fully selected lines as `<a-X>` did.
Adapt existing indentation script to the new behaviour
An indent hook automatically adds whitespace, so it seems prudent to
add the hook to remove unwanted whitespace again. This is what we do
in most languages already.
