Focus fix #4816 (#4897)
Focus fix #4816 (#4897)

Add get_focus_next/previous.
autosportlabs/kivy,KeyWeeUsr/kivy,inclement/kivy,rnixx/kivy,Cheaterman/kivy,KeyWeeUsr/kivy,kivy/kivy,jkankiewicz/kivy,matham/kivy,LogicalDash/kivy,autosportlabs/kivy,jkankiewicz/kivy,LogicalDash/kivy,manthansharma/kivy,kivy/kivy,rafalo1333/kivy,jkankiewicz/kivy,akshayaurora/kivy,janssen/kivy,KeyWeeUsr/kivy,matham/kivy,rnixx/kivy,matham/kivy,tony/kivy,bionoid/kivy,autosportlabs/kivy,manthansharma/kivy,manthansharma/kivy,tony/kivy,janssen/kivy,KeyWeeUsr/kivy,bionoid/kivy,janssen/kivy,kived/kivy,Cheaterman/kivy,Cheaterman/kivy,jegger/kivy,akshayaurora/kivy,Cheaterman/kivy,kived/kivy,rafalo1333/kivy,jegger/kivy,kived/kivy,LogicalDash/kivy,janssen/kivy,rnixx/kivy,rafalo1333/kivy,tony/kivy,jegger/kivy,akshayaurora/kivy,matham/kivy,kivy/kivy,jegger/kivy,LogicalDash/kivy,bionoid/kivy,inclement/kivy,bionoid/kivy,jkankiewicz/kivy,inclement/kivy,manthansharma/kivy