Evas object: Set manual_free to false once obj is detached from canvas.

Evas object: Set manual_free to false once obj is detached from canvas.

This means unref will really free the object instead of just let it
linger in memory once the object is unrefed.
This is also correct because once detached from the canvas, Evas has
nothing to do with the object and it shouldn't need the manual free
anymore.