MMTabline: Add Vim colorscheme / window use tab fill color support

[ychin]

Tabs can now use colors defined by the Vim colorscheme under the TabLine/TabLineFill/TabLineSel highlight groups. There are now 3 coloring modes available: default, automatic (the default mode that generates matching colors using foreground/background colors), and Vim colorscheme. The new mode looks quite nice in some colorschemes and allows user customization, but for some existing ones it doesn't quite look right as they were designed for non-GUI tabs, which is why it's not the default.

MacVim window can now also use the tabline's fill color as the window background color, which allows the title bar to show as a cohesive whole when transparent title bar is set. This creates a nice look especially when the colorscheme is designed for this.

Other tab coloring changes:

• Tabs now support high contrast mode (macOS accessibility setting) now. The default colors will use higher contrast colors, and in all modes, the tabs will be drawn with an outline similar to other system native UI to aid visual differentiation.
• Tabs will also show a dimmed text color when the window has lost focus, similar to other native title bar / tool bar UI elements.
• Fixed default colors to look a little better. Previously the fill color and unselected tab background colors were the same.

[ychin]

Just some visual references for what tabs look like below.

Also, see #1120 for original tabs implementation and also discussion about "transparent title bar" and how it would be nice to pick the Vim colorscheme values and whatnot.

Default colors

I changed the colors a bit. In the original implementation the fill color and unselected tab background colors were the same, so there were no visual borders. I changed it so the fill colors are lighter now and you do see a border of the unselected tabs relative to the background. I'm a little undecided which style looks better as it's a tradeoff between visual clarity vs minimalist look. I decided to go with the differentiated look to make the style more consistent with auto colors (in the Vim colorscheme mode though it's pretty much up to each colorscheme what they want to do).

Light (old):

Dark (old):

Light:

Dark:

Light high-contrast (✨NEW✨):

Dark high-contrast (✨NEW✨):

Automatic colors

This is what we had before. Transparent title bar is set in the screenshots. And also shown is whether we have the ✨ NEW✨ "Window: use tabs background color" option off/on for comparison:

Color: desert:

Color: darkblue:

Color: retrobox (light/dark):

Third-party colorschemes:

Color: solarized (light/dark):

Color: iceberg (light/dark):

Vim colorscheme colors (✨NEW✨ )

This is the new feature that now respects the TabLine/etc highlight groups. Note that some colorschemes look kind of odd (desert) whereas some other ones (iceberg) look very well integrated, intentional and nice; and some (darkblue) just looks kind of mediocre. This is why it's better to just have Automatic Colors as default and the motivated user can set to use Vim colorscheme if they know their colorscheme works well or they are willing to tweak the highlight values.

All the screenshots have "Window: use tabs background color" (✨NEW✨) set to be ON for simplicity's sake.

Color: desert

Color: darkblue

Color: retrobox (light/dark)

Third-party colorschemes:

Color: solarized (light/dark):

Color: iceberg (light/dark):

High-contrast outlines work in auto / Vim colorscheme modes too. Below is solarized (dark) in high-contrast mode: