Bring back rounded tabs and the title bar color being different from the active tab color.

[MathisP75]

I have seen several screenshots of older versions, where the tabs were rounded at the top and where the background color for the title bar was different than the one used for the active tab. This is a regression and I think it should be brought back.

Rounded tabs:

Different title bar color:

These two features should be combined to make tabs look something like the file explorer ones:

[MathisP75]

Here's a rough mockup I made: