Improved filter by tag gallery functionality.

Also added "final" modifier to some missing fields (only in the Mockup code) because inticating that things should not change simply narrows down the possibilities that I (or another programer working on my code) will misinterpret or misuse the thought process which resulted in my code. Or at least it should ring some bells when they now want to change my previously inmutable thing.

Plus marking fields as final forces them to be set by end of construction, making that field reference immutable. This allows safe publication of fields and can avoid the need for synchronization on later reads. (Note that the things a final object references can still change)