Fix C4512 ("'class' : assignment operator could not be generated", MSVC warning level 4) compiler warnings that leak into applications using Urho.

Fix C4512 ("'class' : assignment operator could not be generated", MSVC warning level 4) compiler warnings that leak into applications using Urho.

I would recommend moving to using level 4 instead of the level 3 that CMake uses by default as the default warning level on MSVC in order to catch many quite useful warnings (there will be lots of warnings...).