core: Fix GWT RPC serialization issue in RepoImage

Since no UI code references RepoImage constructor, both
constructor (RepoImage method) and instance initializer
($init method) are marked as dead code by GWT compiler.

In consequence, instance initializer which is responsible
for adding fields such as "private X field = Y;" won't be
part of resulting JavaScript and corresponding GWT RPC
serialization policy.

In other words, if UI code doesn't reference business
entity constructor and we still want to use this entity
in UI code, the entity must not contain fields such as:

  private X field = Y;

instead, the entity must contain fields such as:

  private X field;

and do "field = Y;" assignment within its constructor.
Even though constructor won't execute in UI code, all
fields will still be part of resulting JavaScript.

Change-Id: I7085762877afc8f76090cf18f38bd303c5db2864
Signed-off-by: Vojtech Szocs <vszocs@redhat.com>