Allow scrolling in the history panel when many edits are present

When editing an image with a large number of processing steps, the history panel becomes difficult to use because not all entries are visible at once. It can be cumbersome to review, compare, or return to earlier editing stages when the history grows long.

I would like the history panel to support scrolling, allowing users to easily navigate through all recorded editing steps regardless of the number of entries. This would improve usability when working on complex edits with extensive histories.

An alternative could be to provide a collapsible history view or a search/filter function for history entries. However, adding scrolling support would be the simplest and most intuitive solution.

For images that require many adjustments, the history can become longer than the available panel space. Being able to scroll through the complete history would make it much easier to review and manage editing steps.

Locate the history panel entry point in darktable's C desktop UI and reproduce the issue with an image containing many processing steps. Inspect how the panel lays out its entries, then verify that users can navigate the complete history when it exceeds the available space. Done means all recorded steps remain accessible without obscuring the rest of the panel.