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

src/platform/linux/x11/window.rs

@@ -559,7 +559,7 @@ impl Window2 {
                 MouseCursor::NotAllowed => load("crossed_circle"),
 
 
-                /// Resize cursors
+                // Resize cursors
                 MouseCursor::EResize => load("right_side"),
                 MouseCursor::NResize => load("top_side"),
                 MouseCursor::NeResize => load("top_right_corner"),