Add pop-up window showing the keyboard shortcuts

[szabgab]

Make it display when the user presses ?.

Implements #2607

[ehuss]

Thanks! Would it be possible to update the styling for some of the dark themes? Right now it is unreadable there:

Also, can you resolve the conflicts?

[shenef]

How would a regular user know about the ? shortcut without seeing the list of shortcuts? (Edit: checking a bunch of websites, github was the only one that reacts to pressing ?)

Looking at the code, it seems like the shortcut is only active when shift is being held. On custom keyboard layouts the user might not have to press shift to write ? which in turn would make the shortcut inaccessible. (specifying "custom" here cause I, at a glance, wasn't able to find a language/region specific layout where that is the case)

[szabgab]

I thought if this is accepted then I'd suggest to also add an icon to the menu to show a question mark.

You are right about the shift. I'll have to check how to catch ? in general also on keyboards where it might not require pressing down the Shift key, though maybe we can postpone that to when someone has such a case. (I wonder what happens when CapsLock is pressed and then some presses that key.)