Allow both linestyle definition "accents" and dash-patterns as linestyle...

... for collection, lines and patches.

(code & tests)

see. #2136

This is a minimal patch. It seems to me that the linestyle handling could get some more cleanup.
e.g. in lines.py the drawing is handed over to some `_draw_xxx` methods, those are almost only stubs changing the name of the linestyle
If you put the mapper into one, there is no way to decide which side of the mapping is "authoritative" (useable at the end in the backend). So I decided to divide them into a forward and reverse mapper here. So the mappers can be used without checking if the passed linestyle might be ok for the backend etc.