Update properties.pyx (noted Properties are shallow copies, not deepcopies) (#4572)
Update properties.pyx (noted Properties are shallow copies, not deepcopies) (#4572)

* Update properties.pyx

It's a shallow copy. 

This is important in cases where a mutable property is modified; uppermost levels will not change everything, but nested values will propagate (since it's a shallow copy; not a [deepcopy](https://docs.python.org/3.5/library/copy.html#copy.deepcopy)).

Check this [example](https://gist.github.com/user5061/72585e3ba8259d4038be63e38cecb62f) of mutation would be unexpected.

* Update properties.pyx

bionoid/kivy,akshayaurora/kivy,matham/kivy,aron-bordin/kivy,kivy/kivy,jkankiewicz/kivy,manthansharma/kivy,manthansharma/kivy,akshayaurora/kivy,bionoid/kivy,matham/kivy,kived/kivy,inclement/kivy,jegger/kivy,tony/kivy,kived/kivy,jkankiewicz/kivy,matham/kivy,bionoid/kivy,kivy/kivy,inclement/kivy,Cheaterman/kivy,tony/kivy,KeyWeeUsr/kivy,rnixx/kivy,rafalo1333/kivy,autosportlabs/kivy,Cheaterman/kivy,LogicalDash/kivy,janssen/kivy,aron-bordin/kivy,kived/kivy,janssen/kivy,Cheaterman/kivy,matham/kivy,darkopevec/kivy,akshayaurora/kivy,tony/kivy,rnixx/kivy,manthansharma/kivy,LogicalDash/kivy,aron-bordin/kivy,KeyWeeUsr/kivy,LogicalDash/kivy,darkopevec/kivy,jegger/kivy,jkankiewicz/kivy,darkopevec/kivy,KeyWeeUsr/kivy,manthansharma/kivy,kivy/kivy,bionoid/kivy,KeyWeeUsr/kivy,jegger/kivy,aron-bordin/kivy,inclement/kivy,autosportlabs/kivy,autosportlabs/kivy,janssen/kivy,jkankiewicz/kivy,rnixx/kivy,Cheaterman/kivy,rafalo1333/kivy,rafalo1333/kivy,darkopevec/kivy,janssen/kivy,LogicalDash/kivy,jegger/kivy