KShortcutsDialog: add another, simpler, constructor

Working on porting away from deprecated KShortcutsDialog methods, it looks like the dialog is mostly constructed with the default args (KShortcutsEditor::AllActions and KShortcutsEditor::LetterShortcutsAllowed), so it makes sense to add a constructor that's easier to use than having to use that boilerplate everywhere; and the first constructor is still there for specialised usage where you don't want to show all action types, or not allow single letter shortcuts.

Also fix a code example, the first arg of the first constructor is not a KActionCollection, but rather a KShortcutsEditor::ActionTypes.