[REFACTOR] reverted wording Contact -> Buddy

Not every contact is a buddy ! This patch reverts only code changes but
not the messages that are displayed to the user.

Also merged the BuddyDisplayComposite into the
BaseBuddySelectionComposite before renaming it.

Change-Id: I6435d3988e4cc29c1f9a5e121be9bc9fdd33a036
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1161
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>