Fix cursor behavior on OS X.

Don't implement resetCursorRects: this occasionally hides the cursor
since an empty cursor is assigned to the window rectangle.  Implementing
this method is not required since OS displays the window cursor
correctly by default.

Don't reset cursor mode when window loses focus: once again, OS handles
this correctly, and this means that the window cursor state is restored
when window gains focus again.
Fixed changing cursor while it's outside window.

We need to invoke both [NSCursor set] and [NSView addCursorRect].

First call is responsible for changing the cursor if it's inside the
view; second call is responsible for keeping the cursor the same if it's
outside.
Cleanup of OS X cursor fixes.

Closes #334.
Fixes #254.