Explicit imports in pychan

Gets rid of several star imports in favor of explicit ones.
Also removes a few lines of manually duplicated code between .layout and
.common, only retaining the versions in .layout. Adds comments describing
the situation to .common.
If we decide to not support this any longer, a deprecation hint of some
sorts might be appropriate since I've seen several clients importing all
sorts of stuff from widgets.common.

All in all there is a huge amount of work left for getting this PEP-8 ready.