Improve the look of tabs (mockup)

[korreman]

I use tabs quite extensively, and I think they look a bit inconsistent with the rest of the style.

The top two corners (but only the top two!) are sharp, and there are sharp corners between tabs as well. It should be possible to create a rounded design that looks good and is consistent with the rest of the swayfx style. One way to do this could be as follows:

This design has nice visual consistency:

• All corners are rounded.
• Border around the entire buffer.
• No transparent gaps between tabs.

I'm still unsure about how stacked containers could work though.

[clemencyworld1]

welcomed

[WillPower3309]

I've been thinking of adding a new titlebar style that would go well with this idea. This should be possible thanks to the clipped_region param we've added to scenefx