Automatically add String as a generic type parameter

[ripla]

When a component supports generics, user java.lang.String as the default type and notify the user.

Should work for at least the core Vaadin components from Flow.

[szolo]

I have big doubts about user notification. What we should do? Provide a popup all the time? User will see what kind of type argument is used when he will start writing code. I skipped notification for now, can be easily added later with some thoughts about UX.

Also, String type argument will be used for this Flow components:

ComboBox
IronList
ListBox
RadioButton
RadioButtonGroup

Grid is ignored because it's used mostly with beans.