app: Implement client's keyboard shortcuts

[legoktm]

We may want to add more later, but as a start we should match the client's shortcuts:

• Ctrl+Q: Quit
• Ctrl+D: Download all files in the selected conversation
• Ctrl+Return: Send reply

[legoktm]

There is feedback in #1897 regarding what scopes the shortcuts should be active in that should be taken into account when implementing this.