Support for Unity >=2018.4 and hotkeys

• Fix bug causing event drawer to not work in 2018.4 and up
• Fix missing clear on bool args when making new events
• Add hotkeys for Copy, Cut, Paste, Delete, Add, and Duplicate
• For Unity 2018.3 and up, now use the settings provider for the settings window
• Attempt to preserve the method pointer and component pointer when reassigning target object to an event
• Add package.json file and assembly definition to support the Unity package manager
• Now enable all settings by default