Add annotation support for buttons

[Technosword]

Don't merge quite yet as I am still testing it. This update adds support to callback buttons right within the class where you want them defined. It is pretty cool if I do say so myself!

[Nyvil]

Hello Technosword!

I'm thankful for all your time and efforts!
However, the reason I didn't add callback button support is because of the number of exploits there have been (for example button id spoofing), and I'm not sure that they have been resolved yet. That said if you wish to maybe not push it to stable, but to another branch, called buttons or something, with a note in readme or something that there is said branch, I'll gladly accept your PR (after you give me the go)

Best Regards,
Nyvil