Arrow with tip of arrow = end of feature

[ThHarbig]

Currently, you can make an arrow shape by using a rect and adding a triangle, e.g. in this example. However, the end of the feature is the end of the rect and not the end of the triangle mark.

------->
|-gene-|
vs
-------->
|-gene-|

This could be done by implementing a mark of type "shape" #241 or by implementing a possibility to offset beginning/end coordinates of a rect x and xe by the size of the triangle shape.