Implement changes requested in review

Implement changes requested in review

- Make Peer::_connectedSince, Peer::_buildDate, Peer::_clientVersion
  private, add getters/setters
- Correct parameter naming in doxygen comment for
  SignalProxy::restrictTargetPeers
- Remove SignalProxy::_peers
- Initialize SignalProxy::_sourcePeer to nullptr
- Add Q_UNUSED for peer parameter for CoreSession::changePassword
- Correct indentation in src/qtui/CMakeLists
- Use pragma in src/qtui/coresessionwidget.h
- Fix header ordering in src/qtui/coresessionwidget.h
- Fix header ordering in src/qtui/coresessionwidget.cpp