webadmin: removed caching from EnumTranslator.

* org.ovirt.engine.ui.uicompat.Translator changed to be generic.

* org.ovirt.engine.ui.uicompat.EnumTranslator extends generic
Translator using Enum<?> as type parameter. Doing so there's guarantee
that only enum values are passed to 'get' method, which removed the
need to instantiate EnumTranslator with Class<?> type, because passed
'key' parameter will always have same (class) type as 'type' field.

* Two static factories in EnumTranslator removed and replace by
default constructor.

* Note that key to access resourcebundle is calculated from
Enum.getDeclaringClass() and not Enum.getClass(); when using
polymorphic enum name your resourcebundle keys appropriately.

Change-Id: I3df424d8e7a35ed249b2760da79deba7db31b785
Signed-off-by: Martin Mucha <mmucha@redhat.com>