and stay in it when configuration changes

Closes #370

part of UI changes for #305

Methods for creating, adding and removing forums have been moved to the
`ForumManager`. In order to still handle removing forums properly, a
`RemoveForumHook` has been introduced.

Methods for sharing forums with all current and future contacts have
been removed along with the localGroup where this information was saved.

The `ShareForumActivity` now has the proper label.

The `SessionId` and the `ProtocolEngine` have been moved to the
`clients` package.

This addresses part of #322 and part of what has been discussed in #320.

This commit is a small change that replaces the custom UI elements in
the forum list with a Snackbar and an Toolbar menu.

Addresses one part (2b) of #305

Closes #307

This commit 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 `ContactId`s.

This commit also sneaks in an animation when the 'Share Forum' button
is clicked.

Closes #292

Removes all programatic calls that modify the layout and uses the XML
resource instead.