properties: use weakref for observable list, otherwise, the class will prevent Widget to be released
properties: use weakref for observable list, otherwise, the class will prevent Widget to be released

Conflicts:

	kivy/properties.pyx

ernstp/kivy,kivatu/kivy-bak,denys-duchier/kivy,LogicalDash/kivy,kivy/kivy,LogicalDash/kivy,ehealthafrica-ci/kivy,viralpandey/kivy,ernstp/kivy,vipulroxx/kivy,vipulroxx/kivy,manashmndl/kivy,manthansharma/kivy,manthansharma/kivy,Farkal/kivy,CuriousLearner/kivy,VinGarcia/kivy,Davideddu/kivy-forkedtouch,rafalo1333/kivy,habibmasuro/kivy,jegger/kivy,bob-the-hamster/kivy,manthansharma/kivy,CuriousLearner/kivy,matham/kivy,JohnHowland/kivy,Ramalus/kivy,mSenyor/kivy,inclement/kivy,rnixx/kivy,darkopevec/kivy,ehealthafrica-ci/kivy,thezawad/kivy,Cheaterman/kivy,arcticshores/kivy,bob-the-hamster/kivy,inclement/kivy,adamkh/kivy,kived/kivy,edubrunaldi/kivy,arlowhite/kivy,denys-duchier/kivy,gonzafirewall/kivy,jffernandez/kivy,rnixx/kivy,gonzafirewall/kivy,mSenyor/kivy,JohnHowland/kivy,jehutting/kivy,wangjun/kivy,arcticshores/kivy,bhargav2408/kivy,yoelk/kivy,arcticshores/kivy,bionoid/kivy,angryrancor/kivy,tony/kivy,zennobjects/kivy,darkopevec/kivy,el-ethan/kivy,thezawad/kivy,xpndlabs/kivy,bliz937/kivy,zennobjects/kivy,wangjun/kivy,dirkjot/kivy,bob-the-hamster/kivy,jkankiewicz/kivy,youprofit/kivy,angryrancor/kivy,cbenhagen/kivy,arlowhite/kivy,bionoid/kivy,angryrancor/kivy,bhargav2408/kivy,tony/kivy,niavlys/kivy,dirkjot/kivy,Ramalus/kivy,vitorio/kivy,VinGarcia/kivy,iamutkarshtiwari/kivy,kivy/kivy,jegger/kivy,xiaoyanit/kivy,jegger/kivy,andnovar/kivy,edubrunaldi/kivy,autosportlabs/kivy,dirkjot/kivy,thezawad/kivy,aron-bordin/kivy,jehutting/kivy,inclement/kivy,Shyam10/kivy,denys-duchier/kivy,autosportlabs/kivy,Davideddu/kivy-forkedtouch,KeyWeeUsr/kivy,janssen/kivy,bob-the-hamster/kivy,CuriousLearner/kivy,matham/kivy,bionoid/kivy,bliz937/kivy,eHealthAfrica/kivy,manashmndl/kivy,JohnHowland/kivy,ehealthafrica-ci/kivy,zennobjects/kivy,Cheaterman/kivy,Shyam10/kivy,vipulroxx/kivy,xpndlabs/kivy,manashmndl/kivy,rafalo1333/kivy,hansent/kivy,rnixx/kivy,habibmasuro/kivy,xpndlabs/kivy,janssen/kivy,gonzafirewall/kivy,MiyamotoAkira/kivy,el-ethan/kivy,kivatu/kivy-bak,niavlys/kivy,youprofit/kivy,LogicalDash/kivy,JohnHowland/kivy,mSenyor/kivy,aron-bordin/kivy,angryrancor/kivy,adamkh/kivy,arlowhite/kivy,wangjun/kivy,hansent/kivy,Shyam10/kivy,iamutkarshtiwari/kivy,Shyam10/kivy,MiyamotoAkira/kivy,bliz937/kivy,ernstp/kivy,xiaoyanit/kivy,vitorio/kivy,VinGarcia/kivy,Farkal/kivy,habibmasuro/kivy,jkankiewicz/kivy,eHealthAfrica/kivy,darkopevec/kivy,hansent/kivy,zennobjects/kivy,kived/kivy,cbenhagen/kivy,bionoid/kivy,adamkh/kivy,tony/kivy,ehealthafrica-ci/kivy,KeyWeeUsr/kivy,jegger/kivy,andnovar/kivy,Farkal/kivy,eHealthAfrica/kivy,dirkjot/kivy,kivatu/kivy-bak,jffernandez/kivy,janssen/kivy,arcticshores/kivy,el-ethan/kivy,yoelk/kivy,gonzafirewall/kivy,denys-duchier/kivy,matham/kivy,Davideddu/kivy-forkedtouch,manthansharma/kivy,MiyamotoAkira/kivy,aron-bordin/kivy,janssen/kivy,KeyWeeUsr/kivy,jehutting/kivy,kived/kivy,viralpandey/kivy,niavlys/kivy,niavlys/kivy,Cheaterman/kivy,edubrunaldi/kivy,Farkal/kivy,akshayaurora/kivy,adamkh/kivy,MiyamotoAkira/kivy,Ramalus/kivy,jkankiewicz/kivy,rafalo1333/kivy,jkankiewicz/kivy,yoelk/kivy,Cheaterman/kivy,vipulroxx/kivy,akshayaurora/kivy,jffernandez/kivy,aron-bordin/kivy,andnovar/kivy,jffernandez/kivy,ernstp/kivy,autosportlabs/kivy,viralpandey/kivy,kivy/kivy,LogicalDash/kivy,yoelk/kivy,youprofit/kivy,akshayaurora/kivy,KeyWeeUsr/kivy,eHealthAfrica/kivy,wangjun/kivy,cbenhagen/kivy,Davideddu/kivy-forkedtouch,bhargav2408/kivy,vitorio/kivy,kivatu/kivy-bak,xiaoyanit/kivy,iamutkarshtiwari/kivy,matham/kivy,hansent/kivy,darkopevec/kivy