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!
2d1e072686