fdo#48673 - UI: No keyboard shortcut for "Ignore All" on misspelled word

Also added mnemonic for "Ignore" entry

Change-Id: I044fc0a718a32fe7b502d2c342ba5f09921053ec
Reviewed-on: https://gerrit.libreoffice.org/9908
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>