Integrate image generation with GAPDoc

[markusbaumeister]

Currently, the image generation is done in a chimeric way with GAPDoc. It should be integrated properly with GAPDoc. To do so, active and continued conversation with Frank Lübeck is required to find good design decisions for both our package and all other users of GAPDoc.

A possible change to GAPDoc would be the introduction of an image-tag. Then, the generation of images would be done purely by preprocessing. Some obstacles to be solved along the way:
-> images are pdf for the pdf-version and svg for the html-version. This structure should be kept.
-> not all images are fully specified in doc/Image*, many are only fully generated locally
-> currently, the image names are hashed and only inserted into GAPDoc in the middle of computation

Ideally, this separation would allow other packages to use the TikZ-functionality as well, possibly in a separate package TikZDoc, that acts as a preprocessing to generate images read in GAPDoc.