Implement @-functions in CSS

The CSS parser was extended to support calling registered built-in functions in CSS, e.g. @alpha(color, factor); Functions are called like @-constants which were already implemented previously but must provide comma-separated function arguments enclosed by parenthesis. Additionally the errors returned by CSS parser should be more descriptive. This commit closes issue #8
Merge branch 'master' of github.com:gmc-holle/xfdashboard