Feature 119 scss split

This is the second part of the restructuring of our css code (first part: #415).
In this PR the old css files are split into several small "component" and "widget" files.

Apart from splitting this PR also contains some commits that make use of advanced scss features such as nesting, css3 mixins and variables (mostly for colors). However, since many elements use background images atm we can not make full use of the color variables yet. One of the next big steps will be to get rid of those background images.

There is also some documentation included.

With this PR most dirty work is done and we can begin to merge in improvements and bug fixes that have already been made based on this branch.