Mouse events (#344)

* Explicit mouse-related DeviceEvents

This makes the API more intuitive for common use-cases and allows us
to better propagate platform knowledge of motion semantics.

* Improve event naming consistency

* Clarify axis event forwards-compatibility

* Rename WindowEvent::MouseMoved/Entered/Left to CursorMoved/...

This emphasizes the difference between motion of the host GUI cursor,
as used for clicking on things, and raw mouse(-like) input data, as
used for first-person controls.

* Add support for windows and OSX, fix merging

* Fix warnings and errors on Linux

* Remove unnecessary breaking changes

* Add MouseWheel events to windows and OSX

* Fix bad push call.

* Fix docs, naming, and x11 events

* Remove mutability warning

* Add changelog entry

examples/grabbing.rs

@@ -30,7 +30,7 @@ fn main() {
 
                     WindowEvent::Closed => return ControlFlow::Break,
 
-                    a @ WindowEvent::MouseMoved { .. } => {
+                    a @ WindowEvent::CursorMoved { .. } => {
                         println!("{:?}", a);
                     },