Add a hotkey help dialog with lists of hotkeys

[Yida-Lin]

@cbrnr

Currently there is no way for users to know some hidden hotkeys, such as Ctrl + mouse wheel etc.

I am thinking to add a dialog with all the hotkeys commands listed (analogous to the one below), with an icon next to the zoom in/out. Please let me know what you think.

[cbrnr]

Yes, this is a good idea. In the list, you need two columns: the action description and the keyboard shortcut. For a first iteration, I think the shortcuts could be read-only, but eventually it would be nice to have users enter their own keyboard shortcuts in this list.