Fix some bugs regarding the gradient icon of toolbar and when the brush mode is gradient map

Currently in krita the gradient icon in the toolbar is always a preview of the gradient in 16 bit colour space and the gradient drawn when the brush mode is gradient map is always an 8 bit gradient, they are not affected by the colour space, these two problems are now fixed, but as the foreground colour space is used, when converting the image colour space the effect is not immediate, the foreground colour has to be changed before the effect is visible, I don't know if there is a better solution.