Move axes arrows to reduce whitespace

[ChrisSteel-bio]

Thanks for the cool package. It woudl be great to be able to move the corner arrows in clusterCornerAxes() closer to the UMAP points to reduce white space. I can't seem to find this functionality, only relDist to change the text distance.

[ChrisSteel-bio]

@junjunlab I wondered whether adding something like this would be possible?

[junjunlab]

If you have any ideas or code solutions, please feel free to submit a pull request (PR) on GitHub. Your contributions are welcome.