To start with the search screen instead of the grid set $lens->firstState = "FILTER".
We also join the SupplierID and CategoryID columns using joinLens
so that instead of seeing numbers, we see meaningful descriptions. This
works both when filtering and viewing the grid.
We also configure the categories to show as checkboxes by setting filterParams.
This filterParams setting was configured in dynamic edit mode, then generating
the source code and copying the relevant line into the PHP file.
Easy compared to generating it hand!