Place ell gradient's center to the object's natural center

[Mgushee]

Currently if you fill a shape with a gradient, the
gradient is positioned according to the bounding box.
Thus it may appear off-center in shapes other than
rectangles and ellipses; this is particularly
noticeable with radial gradients in polygons. It should
be at least an option, if not the default, to set the
position of the gradient from the edges or center of
the object itself.

[Guillermo Espertino (Gez) (gespertino-gmail)]

This would also fix another bugs, like the displaced gradient position when you paste the style of an object with gradient fill to another object.
Everytime a gradient fill is applied for the first time on an object, it should be applied in it's center, although it should be easily displaced after that (there are cases, like the radial gradients where it's a need to be able to displace the center).