Changed internal time and data from strings to GDateTime
In this MR are replaced most code where was used strings to store time value with GDateTime. Orage uses internally strings to store time value and these strings decoded mostly by strptime(), but some strptime() implementations seems to have bugs to decode correctly week-day values (see #6 (closed) and #9 (closed)). For future developments, it give more OS independent time handling functions.
This MR fixes issue #9 (closed) and it is also more correct fix to #6 (closed).
In addition it has following fixes or improvements:
- In event list fixed event hiding when event start at 00:00 and end 00:00 in same date (this kind event can be created when setting event as full day, save and then unset full day option). This was probably caused by bug fix 8508.
- Fixed some smaller bugs, like strcmp(..., NULL)
- Replaced time or date passed by labels with object data (g_object_set_data()/g_object_get_data()). This allows to use labels that are not directly related to current time value (like string "Today").