GenCode - Allow overriding the short entity name

GenCode - Allow overriding the short entity name

Before
------

The short entity-name must always be an exact match to the _last_ part of the class-name.

Ex: The class `CRM_Mailing_Event_BAO_Queue` has short entity-name of `Queue`.
This will cause a conflict if any other subsystem has a class ending in `Queue`
(eg `CRM_Contribute_BAO_Queue` or `CRM_Queue_Queue`).

After
-----

By default, the short entity-name matches the last part of the class-name.

But this can be overridden in `xml/schema/*`, eg

```xml
<table>
  <base>CRM/Mailing/Event</base>
  <class>Queue</class>
  <entity>MailingEventQueue</entity>
```

Comments
---------

To see that this change is generally safe (ie does not impact existing entity), I force-regenereated all DAOs

```
GENCODE_FORCE=1 ./bin/setup.sh -g
```

This produced no substantive change in existing DAOs -- because they do not have the `<entity>` tag.