Windows: CursorState improvements (#526)

* Windows: CursorState improvements

Fixes #523

Prior to changing the cursor state, we now check the current grab
state, since it can be invalidated by alt-tabbing and other things.

`CursorState::Hide` is also implemented now.

The cursor name is now wrapped in a `Cursor` struct to allow
multithreaded access.

`Window::set_cursor_state` has been reworked to use
`execute_in_thread`. Two unneeded `transmute` calls were also
removed.

The `WM_SETCURSOR` handler is much more readable now.

`MonitorId::get_adapter_name` has been removed, since it's dead
code and appears to be a relic from 4 years ago.

* Windows: CursorState::Grab no longer hides cursor

`MouseCursor::NoneCursor` has been implemented to allow for
equivalent behavior to the older implementation.

Windows and X11 now have consistent cursor grabbing behavior.
macOS still needs to be updated.

* Windows: Grabbing auto-hides again (for now)

This API needs more work, so let's stick to a bug fix and some
refactoring. However, it now hides using a different technique
than it did originally, which applies instantly instead of after
mouse movement.

CHANGELOG.md

@@ -17,6 +17,9 @@
 - Added the `Window::set_ime_spot(x: i32, y: i32)` method, which is implemented on X11 and macOS.
 - **Breaking**: `os::unix::WindowExt::send_xim_spot(x: i16, y: i16)` no longer exists. Switch to the new `Window::set_ime_spot(x: i32, y: i32)`, which has equivalent functionality.
 - Fixed detection of `Pause` and `Scroll` keys on Windows.
+- On Windows, alt-tabbing while the cursor is grabbed no longer makes it impossible to re-grab the window.
+- On Windows, using `CursorState::Hide` when the cursor is grabbed now ungrabs the cursor first.
+- Implemented `MouseCursor::NoneCursor` on Windows.
 
 # Version 0.14.0 (2018-05-09)