Feature request - Error states on fields

[MightyFo0l]

There is currently no way to detect if something entered is possible or not. For example, in the opacity field any value can be filled in, but only 0, 1 and everything in between is possible.

A solution to this is to give the field a red border if something wrong is entered.

[jeremyckahn]

Thanks for filing this issue! I think this would be an excellent addition to Mantra. I believe that at least some of the work for this would have to be done in https://github.com/jeremyckahn/rekapi-timeline, the timeline library that Mantra uses.

I don't have immediate plans to pick this up, but I'll get to it when I can. If anyone else would like to work on this, please let me know and I'll help out with code reviews and administrative tasks such as making releases.