Reverses QQmlPropertyMap rebase and removed TextureLoader.

QQmlPropertyMap as baseclass doesn't work without additional work, it has been now removed as baseclass from QtCanvas3D for now.
TextureLoader has been removed in favor of TextureImageFactory as that allows easier porting of WebGL based JavaScript code to QtCanvas3D.

Change-Id: I6fb374fd012d7569b9f173b08f4a6f21dcd83abe
Reviewed-by: Tomi Korpipää <tomi.korpipaa@digia.com>
Reviewed-by: Pasi Keränen <pasi.keranen@digia.com>