calendar: Fix "Show all"/"Hide all" button not behaving on expected collections

Before this patch, these buttons acted on all collections, not only the ones under the right-clicked heading. This makes them behave as expected.

NOTE: I am not a fan of the magic number I had to use to get information out of the model. I think this whole feature would be best implemented on the C++ side, but I have no clue where that model data comes from- akonadi stuff is just black magic to me for now.