Validation and actions for View Settings

[doekenorg]

Currently, the view settings do not have a validation system in place. This means the values can be basically anything, or be omitted. To have more fine grain control over the inputs, we'd need validation.

Describe the solution you'd like

• Every settings should at least be able to be: required.
• It should have a validate key which supports a callback, that receives the posted value, and the rest of the settings. Basically how Gravity Forms itself handles it.
• Fields & Widgets should also be able to have front-end validation; but this is secondary if the backend works well enough.
• Validation should also work when changing the form
• An action key could be added with incremental options for the future, but at least have: clear_on_form_change for example.

This solution is just whats off the top of my head, it might be better solved.