display-box: Don't pass event directly to on_key_press_event()

Instead, pass keyval and state.

Since DisplayView corrects keyval, this means that Nintendo DS shortcuts
are now layout-independent.

See a378f4b8
2 jobs for master in 14 minutes and 15 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #392279
flatpak:app

00:10:34

 
  Test
passed #392280
flatpak:tests

00:03:41