forced filters vs default filters

[guruofgentoo]

Some filters are necessary to be present, in order to reduce the amount of data a grid is processing. Otherwise the results could take a very long time to complete. In other cases, they are simply user helpers which should be allowed to be cleared.

What we need is the concept of a "forced" filter, introduced by having is_forced=True in the filter constructor. Forced filters will need to require a default op, and in the UI would remove the possibility of disabling them.

"Default" filters would then need the grid to tell them if they should be processed with the default op. If the user has selected other filters, the default should be allowed to lapse.