Add support for SDL groups

Add support for SDL groups

SDL group is an unordered collection of members where each member is
unique.

Add the following methods to do operations with groups:
-AddMember: adds number of members into a given group. It is possible to
give already existing members to be added but that doesn't have an
effect to group.
-RemoveMember: Remove number of members from a group. It is possible to
try to remove non-existing members from a group without an error.
-RemoveGroup: Remove the whole group along with its members.
-GetMembers: Get all members from a group.
-IsMember: Check if a specific member is part of the group.
-GroupSize: Return the number of members in group.

Change-Id: I9005ffe584aa0872c5a3fab170d0ac37d9b9f30a
Signed-off-by: Marco Tallskog <marco.tallskog@nokia.com>