Hotkey helper overlay

[dlangenk]

Assigning the 1-9 keys to labels is a great way to quickly change labels without a menu. In addition there are multiple other hotkeys now. It would be nice to have a little overview from time to time to quickly see what key sea urchin was again. In addition this would probably allow users to use more of the hotkeys and features that are actually there. I could think of two solutions:

1. I don't think we have used the 'h' key. When pressing the k show a popup which only is visible while pressing the key.
2. We could also have a question mark button next to the annotation tools which shows the same popup while hovering over.

Instead of a popup having multiple tooltips pop up, e.g. above the annotation tool buttons would also work, but I'd prefer the popup solution. But I am also open to refinements of this idea.

[mzur]

Duplicate of #500?