Add uni-form-item helper part

[PatrykSzwer]

I added: FormItemMessagesBuilder - recieves properties that user wants to have form messages and creates view-model object with proper schema. FormItemMessages - View-model of ItemMessages object, contains all methods that end user should use to set, get, and clear messages. FormItemMessage - class that represents message object, which is returned when getting a message. MessageContainer - Container that contains properties schema representations and allows to perform operations on them.

How to use:

1. Add container object to the view-model json: "ItemMessages": {}

2. In view-model code behind:

       static FormItemPage()
       {
           DefaultTemplate.ItemMessages.InstanceType = typeof(ItemMessages);
       }
   
       public void Init()
       {
           this.ItemMessages = new FormItemMessagesBuilder().ForProperty(nameof(this.Word)).Build();
       }

3. Start using it!

   void Handle(Input.Word action)
       {
           if (action.Value.Length == 0)
           {
               this.ItemMessages.SetMessage(nameof(this.Word), "Word cannot be empty!", MessageType.Invalid);
           }
       }

[PatrykSzwer]

I applied all requested changes. Please check.

[PatrykSzwer]

Newly requested changes applied. Please check.