Wayland: rework the event loop & expose readiness signal (#298)

* wayland: don't create a second event_queue

As each EventsLoop has its own context, this is no longer necessary.

* wayland: buffer events rather than direct dispatch

Changes the behavior of the event loop to first internally
buffer the events generated by the wayland handlers, and then
dispatch them to the client's closure.

- It simplifies the event loop logic
- It makes it possible for the user to call window methods such as
  `set_title()` or `set_inner_size()` without causing a deadlock

* wayland: add is_ready() & fix protocol errors

Adds a `is_ready()` method to the windows to advertize
when it is legal to start drawing, and fix a few wayland
protocol mishandling in the process.

CHANGELOG.md

@@ -1,6 +1,8 @@
 # Unreleased
 
 - Uniformize keyboard scancode values accross Wayland and X11 (#297).
+- Internal rework of the wayland event loop
+- Added method `os::linux::WindowExt::is_ready`
 
 # Version 0.8.1 (2017-09-22)