Make various bits of the Apper tool chain optional

This introduces some options in cmake which allows you
to build only some parts of Apper's collection of tools.

Note: Further work along this path might include adding
runtime cmake check modules for e.g. apper-pk-session,
so that can be optionally required by other tools (with
sufficient documentation to support that mode of operation).

CCBUG:377646

Differential Revision: https://phabricator.kde.org/D5194