This now matches how the browser works, where the dropdown is
closed when the window scrolls.
The arrow was also covering the clear placeholder icon, this has
now been fixed and the clear icon is now placed correctly.
This is inspired by https://github.com/ivaynberg/select2/issues/2826
and starts the general transition to using selectors with a lower
specificity. This should make it considerably easier to override
the styles in the long run.
Unfortunately libsass does not support the SASS 3.3 style for BEM
where we can just use `&__m` for defining sub-modules. For now we
are still using a limited amount of nesting, but nowhere near as
much as we did in the past.
This adds decorator support in a very basic way, but enough that
it doesn't take a lot of effort to get it implemented.
This also starts work on splitting things out for theming.
