Fix PYQTDESIGNERPATH mechanism for pydm pip installations?

[peroame]

When installing pydm through pip, the PYQTDESIGNERPATH designer mechanism designer doesn't seem to work, ie the pydm widgets are not visible. I believe this is because the designer needs to read pydm_designer_plugin.py but the pip installation starts from one level below the top pydm directory that contains that file. If this is true, I believe it would be worth fixing pip pydm, if not, please let me know what I'm missing.

Thank you!
Amedeo