Better support for processing dirty/deleted exceptions and `SEQUENCE` of instances

[rfc2822]

Currently, DAVx5 increases the SEQUENCE of dirty instances of recurring events, but synctools increases the SEQUENCE of the main event during the mapping to VEvents.

Instead, we should have that functionality in synctools.

Idea

AndroidRecurringCalendar provides the above-mentioned method (instead of DAVx⁵). It iterates through dirty events and exceptions and creates an extended row with a flag that defines how SEQUENCE should be handled for each event:

• Main event is dirty → main event gets "increase sequence" flag
• Only instance is dirty → instance gets "increase sequence" flag, main event gets "don't increase sequence" flag
• Events that already have a flag are not processed (to avoid increasing the sequence multiple times, for instance on a failed upload).

The AndroidEventProcessor can then generate increased SEQUENCE fields according to the flag.

The flag is cleared on clearDirty(), which then finally increases the locally stored SEQUENCE.

Further improvements

The SEQUENCE should only be increased on important updates, not on attendee status changes.

There's a feature in the Calendar provider that can be used to see what the changes were:

• https://developer.android.com/reference/android/provider/CalendarContract.SyncColumns#CAN_PARTIALLY_UPDATE
• https://developer.android.com/reference/android/provider/CalendarContract.EventsColumns#LAST_SYNCED

[rfc2822]

This topic is also very tightly bound to #126, so I'll close that issue.

We should:

• Process exceptions that are marked as deleted so that they're actually deleted and the main event is marked as dirty instead.
• Process exceptions that are marked as dirty so that the main event is marked as dirty and the exception is unmarked.