add a shortcut overview

[szaimen]

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Since recent enhancements it is possible to

1. Delete a file (ctrl+del)
2. Download a file (ctrl+s)
3. Edit a file (ctrl+e) using a shortcut.

We should think about adding an overview about available shortcuts to the viewer.

cc @jancborchardt and @nimishavijay @skjnldsv on input and maybe mockups.

[nimishavijay]

Universally, the shortcut for viewing all keyboard for shortcuts could be ? which could open a modal with all the shortcuts. This behaviour could be across different apps, and hence also when the viewer is open. ref the issue in server for including keyboard shortcuts