This MR introduces an abstract BaseContactListAdapter which provides
most of the adapter logic. The original ContactListAdapter extends it to
show date and online status of the contacts.
The new ContactChooserAdapter which is used for introductions extends
the ContactListAdapter and adds logic for graying out contacts from
different identities than the currently used one.
A new ContactSelectorAdapter extends the BaseContactListAdapter and
allows to select multiple contacts. It offers a method to return a
collection of all selected ContactIds.
Closes #292 (closed)