gtk3: Fix date picker popup behaviour

The popup behaviour in the custom date cellrenderer and togglebutton popup is very problematic in both wayland and x11 now, and also throws the remaining gtk deprecation warnings. In X11 (at least with Xwayland) it closes the popup without selecting date when choosing a date, and on Wayland there's huge popup positioning and grab handling issues.

If a popover can be spawned as an action from a cellrenderer (I don't see why not), I think we should just move the calendar code into a new popover class and just use that in all cases, both for cellrenderer (PlannerCellRendererDate) and the button one (PlannerPopupButton). Then with a new simpler cell renderer (the text entry didn't work anyways) and a new button one (or perhaps even a more fancy one like gnome-calendar has), after which PlannedCellRendererPopup and co can just get fully dropped finally (other usages of it got ported away from before).