Enables krita to read, convert and save the colour space of grouplayer

Before this patch, if the artist wants to achieve the effect of mixing layers in a different colour space from that of the document, they can only create a new document with the corresponding colour space, edit it and then copy and paste it into the main document, which is very troublesome. This patch allows the artist to convert the colour space of the layer group when converting the colour space of the layer group as well as converting the layer group's own colour space, which allows the layers of the layer group to be mixed in the converted colour space instead of the colour space of the image. This allows the layers in a layer group to blend in the converted colour space instead of the colour space of the image. This patch also adds the ability to read and save the colour space of layer group, and when a new paint layer or layer group is created its colour space will be set to the colour space of the outer layer group.