Refactor event management in Viewer

[dlyr]

PR #776 handles custom events as a map of lamdba
The resulting code looks cleaner than the legacy behavior and we can use it as our new coding style for event management.
e.g.

before

   if ( actionViewer.isValid() )
    {

        if ( actionViewer == VIEWER_TOGGLE_WIREFRAME )
        {
            m_currentRenderer->toggleWireframe();
            eventCatched = true;
        }
        else if ( actionViewer == VIEWER_RELOAD_SHADERS )
        {
            reloadShaders();
            eventCatched = true;
        }
        else if ( actionViewer == VIEWER_PICKING_MULTI_CIRCLE )
        {
            m_isBrushPickingEnabled = !m_isBrushPickingEnabled;
            m_currentRenderer->setBrushRadius( m_isBrushPickingEnabled ? m_brushRadius : 0 );
            emit toggleBrushPicking( m_isBrushPickingEnabled );
            eventCatched = true;
        }
  //...

** after**

    // init somewhere ...
    m_customActions[KeyEventType::KeyPressed][VIEWER_TOGGLE_WIREFRAME] = [this]( QKeyEvent* ) {
        m_currentRenderer->toggleWireframe();
    };
    m_customActions[KeyEventType::KeyPressed][VIEWER_RELOAD_SHADERS] = [this]( QKeyEvent* ) {
        reloadShaders();
    };
    m_customActions[KeyEventType::KeyPressed][VIEWER_PICKING_MULTI_CIRCLE] =
        [this]( QMouseEvent* ) {
            m_isBrushPickingEnabled = !m_isBrushPickingEnabled;
            m_currentRenderer->setBrushRadius( m_isBrushPickingEnabled ? m_brushRadius : 0 );
            emit toggleBrushPicking( m_isBrushPickingEnabled );
        };
    // in handle*Event ...
    auto itr = m_customActions[KeyEventType::KeyPressed].find( actionViewer );
    if ( itr != m_customActions[KeyEventType::KeyPressed].end() )
    {
        itr->second( event );
        eventCatched = true;
    }