Cleanup the styling of the Collection view header

The current header has problems.

1) It assumes that a term always has a header image. In case that image is missing, it looks completely broken.
2) The implementation is a bit complicated, with two extra views used to assemble the header.
3) The fact that the header links are in another view that is absolutely positioned means that the parent (header) div can't stretch to accomodate
additional links (so if the header has a background color instead of an image, it still won't stretch so that all links fit)

Solution:
Remove the "taxonomy title" view completely.
Add a preprocess function to the collection view, get the current tid from the url, fetch the title and background image (or fallback to background color).
Theme the collection view so that the header includes the title and the image / color.
Theme the collection block (providing the links) so that it's positioned relatively, allowing the header to stretch.