Split sunmenu into its own file

Utilizing the good parts of !598 (closed) for Reasons™ that will become clear in the near future.

merged

mentioned in merge request !621 (merged)

By ave on 2023-11-19T20:16:28 (imported from GitLab)

mentioned in commit e8e949ce

By dos on 2023-11-20T02:12:45 (imported from GitLab)

mentioned in merge request !622 (merged)

By dos on 2023-11-20T02:15:42 (imported from GitLab)

mentioned in commit 89a3c5c4

By dos on 2023-11-20T02:33:28 (imported from GitLab)

mentioned in commit 565f7022

By dos on 2023-11-21T00:08:20 (imported from GitLab)