…bserver types

…able and movable

This prevents a hang and/or segfault when trying to lock a moved-from observer.

When moving an observer, we now do the equivalent of a disconnect_all on the source object, and also re-wire those connections onto the target object:
- Insert all connections previously on the source object into the target
- On all connected observers, disconnect them from the source object and re-connect to the target

… while moving

…, disconnect_all is a noop and we don't need movedFrom

…onstructors

Explicitly default/delete copy and move constructors for signal and observer types

Enhanced the move tests and resolved uncovered issues in move_connections_from.
Normalized method names, default statement locations, comments.

use CPM for booststap
use catch2 for unit tests, not yet finisched!

add used but missing includes (include_what_you_use)

mv header to include subdir

…o-signal-slot into feature/add-cmake-support