Don't need to load grunt-cssjanus manually
In grunt cssmin, fix invalid src syntax - this isn't regex :)
For grunt dev build, only clean:all and generate LESS.

Everything else is just getting things ready for production.
Update all main CSS files to contain non-minified versions.

They will instead by generated by `grunt build-prod` when a production
release is built.
Grunt LESS: save CSS files to src, not build.

src is a development copy of the plugin, and as such it requires
compiled CSS to load the stylesheets in the plugin during development.

When `grunt build-prod` is run, the copy:all command will copy the
generated CSS and place it in the build folder. CSSJanus, CSSMin are
then run on the copy of the CSS in the build folder.
Grunt CSSMin: split into LTR and RTL commands

LTR source path is in SOURCE_DIR, but RTL path is in BUILD_DIR as
that's where the CSSJanus command will have placed the file.
Grunt CSSMin: add new RTL lines missed from previous commit
Grunt CSSJanus: rename `dev` command to `core` for consistency with other commands.
Grunt CSSJanus: take CSS files from SOURCE_DIR, not BUILD_DIR.

The LESS command saves the generated CSS into the SOURCE_DIR to allow
the /src/ directory to be a fully-working development version of the
plugin.