Shortcuts Window/Command Palette

### Problem

We currently rely on tooltips and menu items to expose keyboard shortcuts, but:
- there's no centralized way to search actions
- Tooltips only work if you know about tooltips and are using an input that can show them (not touch)
- There may be keyboard shortcuts available that aren't associated with a tool item or menu item

### Proposal

- [ ] A built-in feature to show keyboard shortcuts, either via a keyboard shortcut or via holding down the CTRL key
- [ ] Expose actions that don't have an accelerator?
- [ ] It should probably be automatically populated by adding [action accels](https://valadoc.org/gtk+-3.0/Gtk.Application.set_accels_for_action.html)
- [ ] It should be searchable

### Prior Art (Optional)

VS Code Command palette: https://code.visualstudio.com/docs/getstarted/userinterface#_command-palette

iPad OS/Vision OS keyboard shortcuts window: https://developer.apple.com/design/human-interface-guidelines/keyboards#iPadOS-visionOS

https://docs.gtk.org/gtk4/class.ShortcutsWindow.html