Move and rename server classes which go into the compatibility package
* Use com.vaadin.v7
* Use the same class name as in Vaadin 7
* Use a "vaadin7-" declarative prefix for Vaadin 7 components
Change-Id: I19a27f3835b18980b91a4f8f9464b2adde1a5fd5
The toString() method should not be used on a property to get its value.
Added getStringValue() to AbstractField, Label etc. Using them
where applicable. Added comments to some problematic locations where
Property.toString() is known to be used.
AbstractField.toString() and Label.toString() still return the same
values as before, but it will throw an exception in future revisions.
Migration needed: Replace explicit and implicit uses of
Property.toString() with use property.getValue() and its
string representation.
Alternatively, use AbstractProperty.getStringValue() and
AbstractField.getStringValue() instead of Property.toString() during
migration.