`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
Some languages have a trim-indent command but don't use it (for no
apparent reason). Make them trim trailing spaces when exiting insert
mode, like most other languages support scripts do.
This commit allows using the <semicolon> expansion in commands, instead
of `\;`.
It makes commands look more elegant, and prevents new-comers from
falling into the trap of using <a-;> without escaping the semicolon.
