change getLastIdUsed to lastUsedContactMethod

Changes the function to return the pointer to the last used
ContactMethod. This is more generic than returning the id, since
the caller can then decide what info they need themselves.

Drops the "get" from the function name to follow LRC/Qt
convention: getters don't begin with "get".

Makes this getter public as its useful for other purposes as well.

Change-Id: I8eb57c8d830e84e76365311870aaaf10dc5739a6
Reviewed-by: Nicolas Jäger <nicolas.jager@savoirfairelinux.com>