Add multistroke classes to factory_registers.py
Merge pull request #2142 from bionoid/factory_registers

Add multistroke classes to factory_registers.py
uix:EffectWidget use  property to set the background of the fbo, base EffectWidget of of RelativeLayout instead of BoxLayout.
uix:EffectWidget doc for background_color
Fix issues that broke doc build
Further doc style cleanup is on the todo list for now
Merge pull request #2144 from bionoid/docfix

Fix issues that broke doc build
Remove unused imports. Was leading to crashing modules that used widgets using behaviors, when launched through config.
Merge pull request #2143 from kivy/effect_widget

Effect widget
Fixes #2140  Syntax error

Syntax error in print statement did not work in Python3.
Merge branch 'issue_2140'