Fill remaining space in top/bottom tab layout

[Nerixyz]

When having many tabs open, where the tabs span multiple lines, there will (almost) always be some space on the right side of the tabs. With multiple lines, the lines (usually) aren't equally wide, thus the remaining space isn't equal. It would be great if the tabs could wrap as they do now but expand to the width of the window.

From

┌─────┬─────┬─────┬──────┐
│ foo ┊ bar ┊ baz ┊      │
├─────┴┬────┴┬────┴┬───┬─┤
│ foo2 ┊ bar ┊ baz ┊ + ┊ │
└──────┴─────┴─────┴───┴─┘

To

┌────────┬───────┬───────┐
│  foo   ┊  bar  ┊  baz  │
├───────┬┴─────┬─┴───┬───┤
│ foo2  ┊ bar  ┊ baz ┊ + │
└───────┴──────┴─────┴───┘

A small refactor of the tab layout would probably be nice too.

[8thony]

Eventual, some related stuff.
I list it here, but don't mind me if it is out of scope of anything you would like to do / fit within this refactor.

• On small window not all information is displayed correctly #4007
• Tab scrolling doesn't work when mouse between tabs #2970
• Allow scroll to work even when tabs are hidden #2916