Sort Task Lists using calendar-order

[marbetschar]

This PR will eventually fix #169 by adding support for the calendar-order property. Because this property was added in a newer Evolution Data Server version than shipped with the Ubuntu 20.04 LTS base, using Flatpak is a must.

This way, we can package a newer version of the needed evolution-data-server and ship it within the Tasks app.

[marbetschar]

The necessary code is in place, but right now throws error: E.util_source_compare_for_sort' is not available in libedataserver-1.2 3.39.2. Use libedataserver-1.2 >= 3.40.

Therefore we need to wait for the EDS 3.40 release and adjust the io.elementary.tasks.yml accordingly.

[Marukesu]

Therefore we need to wait for the EDS 3.40 release and adjust the io.elementary.tasks.yml accordingly.

A problem can be the DBus interfaces, we need them in the same version of the host, since we use the online accounts plug to login.

[marbetschar]

@Marukesu I merged EDS v3.40 and E.util_source_compare_for_sort gets executed in MainWindow.vala:323. But it does not seem to have any effect.

How can I verify if this is related to the locally installed EDS version (not the one in the Flatpak) not populating the calendar-order field?

Nevermind. Obviously I can just print the field 🤣

[marbetschar]

Alright; the E.SourceSelectable.order field is always 0 - so the new EDS function E.util_source_compare_for_sort uses the display_name for sorting.

Therefore this PR is ready to be merged. However, if we really want to make use of the new calendar-order property, the non-Flatpak version of EDS needs to be updated.