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
Also fix some small issues in Table declarative:
* Write null property values as empty strings instead of NPEing
* Read/write item ids from/to <tr item-id="...">
Change-Id: Ieccc3f49c5021f8a4a50d4ea671f9086ad8f997c