This patch fixes an issue when creating new events in week view the week
after the daylight saving change.
The problem was in the calc of the selected date, that's done couting
the 30 minutes slots and adding that minutes to the week_start datetime.
When there's a daylight saving change, the real date to select should be
adjusted. This patch adds a new util function to make it easy to add
minutes ignoring the tz so we can make this calc directly with
gcal_date_time_add_floating_minutes.
This also includes a new test file with some checks for the new util
function and spring / autumn changes.
This new function could be used in other places to fix specific issues
that could be caused by the same logic error, calculate a selection or
event position based on GDateTime operations.
Fix https://gitlab.gnome.org/GNOME/gnome-calendar/-/issues/482
Daniel Garcia Moreno