[Tasks] Add Support for Task Window Element Groups (#3902)

Elements that share a group are placed in the same list section and
separated from other groups with a divider.

Live appears to only use this for optional elements in some tasks and
when used each optional always gets its own group. This might indicate
it's done automatically under certain criteria to ensure optionals are
never grouped with non-optionals regardless of index.

Since groups are available in captures and there's very few tasks that
use this, we don't need to worry about trying to replicate any automatic
behavior since this allows more customization.

common/tasks.h

@@ -75,6 +75,7 @@ struct ActivityInformation {
 	std::string      zones; // IDs ; separated, ZoneID is the first in this list for older clients -- default empty string, max length 64
 	int              zone_version;
 	bool             optional;
+	uint8_t          list_group; // element group in window list (groups separated by dividers), valid values are 0-19
 	bool             has_area; // non-database field
 
 	inline bool CheckZone(int zone_id, int version) const