Fix issue where grid stores loaded only 20 records even if pagination were disabled

For now, if you disable pagination in a `MODx.grid.Grid` derivative, the store is limited to 20 records (either by L. 104 and the `modObjectGetListProcessor` processor, setting its default limit to 20).
So there is no way to disable the limit.

With this fix, you can "disable" any limit by setting your baseParam.limit to 0 (else, it will still default to the 20 defined in the processor, according you use a `modObjectGetListProcessor` derivative).