Icons: optional meta variables

[mfranzke]

    I like the idea making this optional, but in that case we could as well define it as a fallback to the CSS Variables reference on the following code lines:

https://github.com/db-ui/base/blob/cf16a3bd1e2078e5bf8209dbcc44884bc112c5b2/scss/icon/_icons.helpers.scss#L38L40

Originally posted by @mfranzke in #168 (comment)