window_x11: fix python3 TypeError
window_x11: fix python3 TypeError

Encode Window.title as byte-string before attempting to set the x11 window
title. This avoids "TypeError: expected bytes, str found" when using the
x11 window provider with Python 3.

matham/kivy,matham/kivy,rnixx/kivy,KeyWeeUsr/kivy,matham/kivy,bionoid/kivy,kivy/kivy,KeyWeeUsr/kivy,akshayaurora/kivy,kivy/kivy,Cheaterman/kivy,LogicalDash/kivy,LogicalDash/kivy,inclement/kivy,matham/kivy,Cheaterman/kivy,Cheaterman/kivy,rnixx/kivy,akshayaurora/kivy,kivy/kivy,KeyWeeUsr/kivy,inclement/kivy,KeyWeeUsr/kivy,akshayaurora/kivy,bionoid/kivy,LogicalDash/kivy,rnixx/kivy,Cheaterman/kivy,bionoid/kivy,LogicalDash/kivy,inclement/kivy,bionoid/kivy