Add Preview/Handle structure similar to WPF for filtering TextBox input

[profexorgeek]

This discussion started due to a crash in TerraScriber when the user enters non-numeric characters into a "map size" Textbox, causing the app to crash.

While this issue is a custom code problem and fixable, it triggered a discussion on built-in filtering capabilities. WPF has a system with the concept of Preview and Handle events that allow developers to react to text as it's being entered, filter it, and note that it has been handled.

Forms should implement something similar that offers this flexibility without being opinionated or enumerating every type of filter that users might want.

[profexorgeek]

Discord discussion link:
https://discord.com/channels/586997072373481494/819954682029277185/1333495118501773312

[vchelaru]

This was already implemented, but it was not working for pasted text so I fixed that here:

vchelaru/Gum@0ac7b95