Update tab-bar and tab-line style

[gsingh93]

While https://github.com/bbatsov/zenburn-emacs/pull/371 was meant to be a non-controversial change, making the tab-line style consistent with tab-bar, this PR updates the styles of both to look "better" (in my opinion). The changes include:

• Make the tab-bar font size slightly bigger to improve readability
• Add padding around the tab text
• Change the box style from released/pressed button to nil, for a more flat, modern look.
• Removed the separators between tabs (note that for some reason, the only way to do this and preserve the padding with tab-bar is to use a zero-width space)

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• [X] The commits are consistent with our contribution guidelines
• [X] You've added a before/after screenshot illustrating visually your changes.
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• [X] You've updated the readme (if adding/changing configuration options)

Thanks!

[gsingh93]

I'm not sure if I'm setting the tab-bar-separator and tab-line-separator variables in the right spot. When I load emacs for the first time, tab-bar-separator is set but tab-line-separator is not. But if I run M-X load-theme RET zenburn, then both variables are set. It's probably related to the loading order, where tab-line-mode overwrites this variable when it's loaded, which is after the theme. I'm not sure how to do this in an order-independent way.

[gsingh93]

There's a weird issue with tab-line groups. Here's how it looks at first:

Hovering over the group, you can see some of the hover bleeding over into the buffer name:

Moving the mouse off of it leaves that discoloration there:

And then after hovering over the buffer name and moving the cursor away, things go back to normal, like the first image.

[bbatsov]

No idea about this issue - never used tab-bar, but I assume it's using some overlays to simulate the tabs and I've seen plenty of glitches with them.