- Explicitly set the SignalWatcher event queue. Without this, the watcher will
  publish events to the fast queue, resulting in resize bugs for certain
  terminals(#2322).
- Set `async = false` to the `remote_ui_attach` handler(It was a deferred
  before, this is the new equivalent)

This adds a redraw notification "update_menu" which is sent whenever
Vim's menus are changed by the :menu command and friends.

API functions exposed via msgpack-rpc now fall into two categories:

- async functions, which are executed as soon as the request is parsed
- sync functions, which are invoked in nvim main loop when processing the
  `K_EVENT special key

Only a few functions which can be safely executed in any context are marked as
async.

This is a port of my original contribution to Vim, added in 7.4.687
(https://github.com/vim/vim/commit/v7-4-687). The TUI code has been
heavily refactored (see esp. 25ceadab37edba13f5afa78d8b4723da03ef35f0),
so this required some translation, but the logic is the same.

Currently, there are two functions in the UI API that are called when
the mode changes: insert_mode() and normal_mode(). These can be folded
into a single mode_change() entrypoint which can do whatever it wants
based on the mode it is passed, limited to INSERT and NORMAL for now.

We already use wrappers for allocation, the new `xfree` function is the
equivalent for deallocation and provides a way to fully replace the malloc
implementation used by Neovim.

Switching cursor off is only necessary in two occasions:

- When redrawing to avoid terminal flickering
- When the editor is busy

The first can now be handled by the TUI, so most calls to ui_cursor_off can be
removed from the core.

So, before this commit it was only necessary to switch the cursor off to notify
the user that nvim was running some long operation. Now the cursor_{on,off}
functions have been replaced by busy_{stop,start} which can be handled in a
UI-specific way(turning the cursor off or showing a busy indicator, for
example).

To make things even more simpler, nvim is always busy except when waiting for
user input or other asynchronous events: It automatically switches to a non-busy
state when the event loop is about to be entered for more than 100 milliseconds.

`ui_busy_start` can be called when its not desired to change the busy state in
the event loop (As its now done by functions that perform blocking shell
invocations).

Make the error messages more precise and uniform.