鼠标进入了某个窗口（仅限 Editor 视图）。
用户将鼠标悬停在了某个窗口上，并且没有按住任何鼠标键。该事件被发送了一次，因为鼠标进入了该窗口。
Note that this event is only sent in the Editor for EditorWindow windows which have EditorWindow.wantsMouseEnterLeaveWindow
set to true. Mouse enter or leave window events are never sent in the games.