Backtracking on form

[Bartheleway]

Hi,

This is my trial to fix #1127 (and is also part of #1117)

Here are the main changes :

• Rewrite paper-input with new syntax
• Rewrite paper-autocomplete with new syntax
• Rewrite paper-form with new syntax
• Get rid of parent & child & validation mixin
  • I deleted the files as they are not used anymore but we can keep them for compatibility with old projects
  • Mixins are deprecated so this is quiet a good news => easier to migrate to octane version
• Custom validation doesn't handle custom attribute
  • I didn't see any usage for this & I changed the test accordingly to show how to replace usage
  • This was simpler to migrate to new syntax (and octane version maybe), we can try to keep it if really needed
• I fix some tests related
• I tried to improve the documentation

The tests runs kind of correctly (I didn't care about non related errors).
It seems that backward (3.12) compatibility is an issue, any ideas on how to solve ?

I didn't find how to write a proper test for backtracking error but I confirm this fix the issue on my project and the test I wrote is really similar to my use case.

Feel free to comment & ask for modifications.