This ensures slide with `Correct UVs` enabled, keeps UV's (any loop custom-data) contiguous. Adds 2 key functions `BM_vert_loop_groups_data_layer_create` and `BM_vert_loop_groups_data_layer_merge` They work by collecting matching loop custom-data around the vertices loop-fan, and merging the custom-data after edits are made. Thanks to @mont29 for review!