Modified the delta calculation function to resubmit incidences that failed to upload
There are several commits, mainly cleaning or correcting minor errors. There is one major though about not listing the modified incidences with the mKCal function. Thinking about this, I think it's not needed since we already list all the stored incidences and we can from this list apply the rule to see if the incidence is modified or not. This simplifies much the delta calculation in my opinion.
The last commit is the purpose of this MR. It is there to ensure that incidences that failed to upload will be retried on next sync. This is an issue that we are fault tolerant. Before, the sync time of the notebook was not updated on any error. So the same modified, added and deleted incidences were retried automatically on next sync. Now, the sync date time is updated, and these failing incidences are not listed anymore as deleted or modified (because their deletion date or modification date are earlier than the last sync date time). To solve this is quite simple with the recent additions to mKCal. One just need to list all deleted incidences instead of only the latest ones. For modified it's a bit more tricky at first sight, but in fact, since we flagged the failing incidence, we just need to retry the flagged incidences...
If the upload error was transient like a tunnel in the train at a bad moment, then it should be fine on next sync. If the error is more permanent like a server that don't want to receive modifications, then, well it will fail again but that won't stop the sync process and the user is warned anyway that this event is problematic.