[DataBind] Add support for casting value when accessing it

[titouanmathis]

The DataBind component does not have yet a special handling of values, except for checkboxes and selects.

It could be useful to be able to define the type of the value that should be returned when invoking the get() method or reading the value property.

If we are dealing with inputs of type number for example, it can not be used as follows:

dataBindInstance.value += 1;

[titouanmathis]

We have 2 options :

• add a type option to the components, allowing to specify the type of a group manually
• add valueAsNumber, valueAsBoolean, etc. getters and setters to be able to use them only where needed without having to configure the type everywhere