Add ability to set as default assist app

[pokegamer5547]

Setting as default assist app would come in real handy when we want to clear clipboard ASAP.

[DeweyReed]

I'll check it later when I have some time. Thank you!

[DeweyReed]

https://github.com/DeweyReed/ClipboardCleaner/releases/tag/1.6.0