Niels De Graef authored Sep 08, 2019 and Niels De Graef committed Sep 25, 2019

Currently, there is no quick way to find whether and element is already
part of a list store, except for manually writing a for-loop and calling
`g_list_model_get_item()` and breaking when you find the item.

This is mostly just a small API addition to support this use case.

Fixes #1011