Remove diagnostic_style_id and use vis.ui:style_push to obtain one.

This allows external code to reuse the select method.

Example visrc.lua:
	local lspc = require('plugins/vis-lspc')
	local choice = lspc:select({'A', 'B', 'C', '6'})

made the documentation for `show_message` clearer

Remove broken autoscrolling.
Add commands to control a code/navigation window pair.
Support multiple navigation window pairs.
Improve default navigation window keybindings.