Always update contexts on vtd#view#Enter()

This was causing a bug where toggling contexts (with '*') wouldn't update the
system's contexts.
Add "exclusive context" concept

If set, only that context will be shown; all others will be neither included nor
excluded.
Add '!' keymap to toggle exclusive context
Context highlighting when one context is exclusive
Prevent contexts from jumping around:

Use ' ' instead of '' to mark a bare context.
Non-'!' context actions clear exclusive context
CyclableOption.Setting(): give setting's name
Keymap.display(): a human-readable description
Add s:show_help: a setting whether to show help
Split enter() back into two parts

It was only used twice, and one of those uses was wrong: I need to set up the
buffer after I enter, but before I display.  (I noticed that switching to
NextActions view with help already open wouldn't show the special keymaps.)
Add VtdView.help() to display help
Highlight <C-...> keymaps