Top menu doesn't respond to alt-key

Nobody has claimed this yet.

## Use case

I'm used to the top-menu of an application always being available through the alt-key and a letter. Alt does seem to be working, but the target is the side toolbar, not the top-menu.

## Suggested solution

An option on the preferences menu to select whether alt applies to the side toolbox or top menu.

### Possible drawbacks

Using another person's PC, behavior might not be as expected.

### Possible alternatives

Add a shortcut to the main menu to the side toolbar with its own letter so there's a keyboard sequence to reach it (m doesn't seem to be taken).
Pressing alt-m would act like clicking the "File" menu, after which arrow keys could reach everything else on the menu.

Ctrl-alt-letter to access the top menu instead of the side toolbar.

If there's already a standard Linux way to access the top menu, maybe add a "coming from Windows" cheat-sheet to the help menu that mentions it.

Start by inspecting the preferences menu, side toolbar, and top menu entry points mentioned in the report, then determine how the current Alt-key handling is wired. Confirm the desired behavior and whether it should be configurable with maintainers; done means the selected keyboard path reliably reaches the intended menu and its scope is documented.

Written by the indexing model from the issue text.