Show keyboard shortcuts as hovertext over the action buttons.

[eibhear-from-athlone]

Is your feature request related to a problem? Please describe.
I would like to more easily learn the keyboard short cuts for the various actions in Commafeed.

Describe the solution you'd like
I suggest that when a mouse is hovering over a button, a hover-text bubble is shown giving the keyboard shortcut that could be used instead. For example, when hovering the mouse over the "Mark as unread" button, a hover-text button could show up with "Shortcut: M", or just "M". When hovering over the "Open Link" button, it would read "V". And so on for all actions that have keyboard shortcuts.

Describe alternatives you've considered
The only alternative that I can think of is to learn the shortcuts from the list on the separate "About" screen.

Additional context
No other additional context from me, except to say that I continue to love Commafeed.