[#549] allow non physic objects to be added in the quadtree
[#549] allow non physic objects to be added in the quadtree

this fixes mouse event detection on non-entities (like the debug
panel), and test for a `body` existence is anyway done prior to
collision check

rlugojr/melonJS,melonjs/melonJS,melonjs/melonJS,rlugojr/melonJS,rlugojr/melonJS