Add service::get_{adjacent,conflicting}_meetings

[bkircher]

Background:

Using additional properties, a user can use calendar_item::get_adjacent_meeting_count and calendar_item::get_conflicting_meeting_count to get the number of conflicting, or adjacent meetings, respectively. However, there is currently no way she can retrieve those meetings.

Idea:

Add std::vector<calendar_item> service::get_{adjacent,conflicting}_meetings(const calendar_item&) const member-functions, returning only those calendar items.

Note:

Be careful with xml_subtree::{get_node,get_value_as_string} when implementing this feature as they use depth-first and we have a nested XML structure in that <GetItem/> response.