[libsocialcache] Add e-tag support to CalDavCalendarDatabase

Add eTags() and insertETags() to the public API.

Also add removeIncidenceChangeEntriesOnly(), which will not remove
etags from the database, unlike removeEntries().
[libsocialcache] Bump lib version
Merge pull request #49 from nemomobile/caldav-etags

[libsocialcache] Add e-tag support to CalDavCalendarDatabase
[libsocialcache] Add internal EntryRemovalMode enum and remove some debug
[libsocialcache] Ensure queries in dropTables() succeed

Finish user_version query before calling dropTables(), else db is still
locked when dropTables() tries to execute additional queries.
Merge pull request #50 from nemomobile/finish-query

[libsocialcache] Ensure queries in dropTables() succeed
[libsocialcache] Bump version
[libsocialcache] Initial VK database support
[libsocialcache] Improve VKNotificationsDatabase API
[libsocialcache] Added QML model for VK posts
[libsocialcache] Use string instead of int for 'Type' value in VK notifications database