Fixes #1478. Adds validation to field. Values must be in the store

And again, can be much shorter, like @dnahodil suggested before, for instance:
```
if(value != "") {
    var val = this.getRawValue();
    var rec = this.findRecord(this.displayField, val);
    if(!rec) {
        this.markInvalid(true);
        return false;
    }
}

return true;
```