gui/calendar-list: Turn row into a checkbox

Using a GtkCheckButton widget as a child as we've been doing was a little bit too much, considering it was overachieving its use case. Given the nature of a button, the widget was also capable of being targetable, focusable, and activatable, which forces a keyboard user to tab an additional time to get to the next row in the list.

This patch introduces a new method by turning the entire row into a check button widget, with an image widget that becomes visible or invisible, which addresses the issues raised above. This also avoids binding properties bidirectionally, as the image is only used for decorative purposes.

Fixes !542 (comment 2459629)