extend FormGroup and FormPanel to accept all other types of Widgets

[safibarri]

The idea is to extend the FormGroup and FormPanel to accept other types of Widgets
since i needed to include panels inside that can be shown or hidden based on other input values

[hohwille]

I have added a generic method to add inputs to any kind of mutable composite/panel. This allows to add them to a regular UiVerticalPanel. However, FormPanel still has its unique features such as data binding and layout options so it still needs to be changed.
My plan is to change the API such that it is like any other mutable panel (e.g. UiVerticalPanel):

addChild(UiRegularWidget); // will behave like in a `UiVerticalPanel`
addChild(UiInput); // will behave like it currently does in `UiFormPanel`

Hope this plan makes sense and will finally resolve this issue.