- addressed review feedbacks
- documentations

Enabled states now work. E.g. a mapped Touch Bar button using "vmenu
TouchBar.DoStuff <nop>" will not show up in normal mode.

Also support specifying default Apple template icons. E.g. "an
icon=NSTouchBarListViewTemplate TouchBar.ShowList <Nop>"

Remove default TouchBar buttons as there were too many of them and most
of them are unlikely to be heavily used as there are direct Vim command
equivalent. Instead just add a single fullscreen toggle button. This can
be changed later.