Allow specify required property on DesignTypeParameterAttribute

[unaizorrilla]

At this moment, the metadata for all toggle properties does not allow to specify "Is Required or Is Optional" and this is very usefull for Esquio UI. Currently, on the UI, the user can set null values on toggle properties ( string or numeric properties ) and this missconfiguration could be confusing for the user on Esquio UI and also reduce the performance on toggle evaluation (due throw exceptions)

The steeps to solve this kind of issues could be

• Allow a new property on DesignTypeParameterAttribute to specify if the property is required or not
• Modify the Esquio UI to add "null" validation to required properties
• Modify all toggles to be more defensive on null parameter properties