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
The linestyle drawing functions, their names being the values here, are removed, as linestyles are handled in the backend code