This functionality no longer makes sense inside Moodle now that we have
external tools like stylelint to help us with this. It was not able to
be used by any core themes.

The code was always marked as not for external use.

Invalid CSS with extra } symbols, which previously (by fluke)
worked, was broken by recent chunking improvements. This change
makes the chunking code robust against this (stupid) situation.

The affected code areas are not supposed to be used in any add-ons,
the backwards compatibility should not be a concern for us.

This commit makes the following changes:
* Tidies up coding style.
* Tidies up phpdocs.
* Improve CSS optimiser unit tests to cover browser hacks.
* Improve backgound style consolidation.
* Improve border style consolidation.
* Fixed optimiser to handle browser hacks like *zoom.
* Improved @ rule unit tests.
* Fixed @rule breakages caused by OS specific rules.
* Added more unit tests to cover a bit more CSS3.

I am pretty sure this patch will now enable to optimsier to
work on bootstrap based themes should developers not wish
to compress thier compiled less CSS.

ETag value must be double quoted, http://tools.ietf.org/html/rfc2616#section-3.11.
More details about potential impacts in MDL-39832: shortly, at least when using
HTTP Partial GET Requests (HTTP Status: 206) plus X-Sendfile HTTP Header.
In this commit, the impact is to be RFC2616 compliant when using the ETag Header.

Credit goes to Hubert Chathi, thanks!

MDL-33468 css_optimiser: Fixed aggressive optimisation so that it only combines siblings and doesn't reorder