Allow users to choose how to open messages on double-click

[eriksid]

Is your feature request related to a problem? Please describe.

Yes, the current behavior of double-clicking opens the system in full screen, which might not be the desired action for all users. This lack of flexibility can be frustrating for users who prefer different behaviors.

Describe the solution you'd like

I propose adding a configuration option to customize the double-click behavior. This would allow users to choose from several options:

• Default: Does nothing on double click.
• Open without split: Opens the message in the reading pane without splitting the screen. It should include functionalities for replying and labeling, exactly how the existing reading panel experience.
• Open in a new tab: Opens the message in a new tab, allowing users to keep the message list visible. Similar to "Open without split," it should have options to reply and label.
• Open in full screen: Maintains the current behavior (introduced in commit 68045f5) of opening the message in full screen.

Describe alternatives you've considered

N/A (This section is best used if there were other potential solutions considered but ultimately rejected)

Additional context

This feature would improve user experience by providing flexibility and customization for how users interact with messages.

Here are mockups to illustrate the proposed "Open without split" and "Open in a new tab" functionalities:

• Open without split mockup:
• Open in a new tab mockup:

I am enthusiastic about this feature and would be happy to develop and submit a Pull Request for its implementation if there's interest from the development team.