Add option to make repeated fields ArrayCollections

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Currently optinal or required fields may be watched using data bindings. This 
does not work for repeated (array) fields, since arrays send no events.

A solution for this would be a field option "as3_array_collection":

repeated int32 foo = 1 [(as3_array_collection) = true];

This option changes the type of the repeated field to ArrayCollection.

This enables the use of the field in a data binding as source for e.g. a spark 
list, that will update as soon as the field on the message is changed.

Original issue reported on code.google.com by jan.bo...@gmail.com on 12 Jul 2011 at 9:41