Rework the tabbed user interface

[Jacalz]

Checklist

• I have searched the issue tracker for open issues that relate to the same feature, before opening a new one.
• This issue only relates to a single feature. I will open new issues for any other features.

Is your feature request related to a problem?

The current interface with tabs has been there from the start. It is not necessarily bad but we should consider updating it to see if we can get something nicer looking that works better.

Describe the solution you'd like to see.

An initial idea:

• Add a menu button to the top right of the window.
  • Remove the settings and about tabs from the tab bar and open them from the menu.
• Show the send and receive tabs more clearly. Maybe as two equally sized tabs?

A bit similar to Warp but not in the header bar: