docs: Remove shortname references.

In addition, align the removal notice next to 'key' with other
such notices (options.txt).
runtime: Cleanup refs to removed options

- {no,inv,}shortname
- {no,inv,}sn
- cm, key (remnants of cryptmethod)
Merge pull request #1623 from Pyrohh/cleanup-shortname

Docs: Remove 'shortname' references
doc: Misc. improvements to nvim-related docs

- Improved wording in a few places for clarity
- Various capitalization/grammar fixes
- Change references to Neovim as 'editor' to 'Nvim'
- Be consistent regarding utilization of vim's documentation features,
  e.g. unnamedclip -> |unnamedclip|
- Reflowed all changed paragraphs accordingly

- Add spaces before parentheses
- Remove trailing whitespace
- Standardize single spaces after periods. Vim's docs use two for the
  most part, but Nvim's use one mainly, so just follow Nvim's conventions
Merge pull request #1624 from Pyrohh/doc-fixes

doc: Misc. improvements to nvim-related docs
deps: Update lua client
msgpack-rpc: Allow registration of handlers by other modules
ui: Add abstract_ui termcap and split UI layer

This is how Nvim behaves when the "abstract_ui" termcap is activated:

- No data is written/read to stdout/stdin by default.
- Instead of sending data to stdout, ui_write will parse the termcap codes
  and invoke dispatch functions in the ui.c module.
- The dispatch functions will forward the calls to all attached UI
  instances(each UI instance is an implementation of the UI layer and is
  registered with ui_attach).
- Like with the "builtin_gui" termcap, "abstract_ui" does not contain any key
  sequences. Instead, vim key strings(<cr>, <esc>, etc) are parsed directly by
  input_enqueue and the translated strings are pushed to the input buffer.

With this new input model, its not possible to send mouse events yet. Thats
because mouse sequence parsing happens in term.c/check_termcodes which must
return early when "abstract_ui" is activated.
msgpack-rpc: Add remote_ui module

The remote_ui module is an implementation of the UI layer, and it attaches UI
instances that redirect redraw notifications to connected clients.
test: Add screen test facility implemented as a remote UI