Show keyboard shortcut in action toolbar tooltips

I tried to make these similar to how it works in KXMLGui, which adds them to an existing tooltip. This means the actions document themselves, in the expected way: "I want to keep doing this action, let me hover over it to do it faster!"

One other related change I had to make was changing when the tooltip is shown. Now a tooltip is shown for all display modes, not just IconOnly. This is so icon-beside-text actions like the ones seen in KInfoCenter get this tooltip.

I also made sure the (bad-anyway-case) of an action without text but with a shortcut is handled, and with an action with neither.