Improve keyboard navigation of the App

[legoktm]

Description

This is a follow-up to #1897 + #3062

Ideally it would be possible to navigate through the entire app just using the keyboard, for the users that want it. In some cases tab navigation should already be possible but shortcuts can be faster, e.g. an explict focus on the reply box shortcut instead of tabbing through the whole conversation.

E.g.

• Ctrl+Up and Ctrl+Down to move through the source list
• Ctrl+S to star the active source (s for star/save)
• Ctrl+R to jump to the reply box