LibGUI: Only consider starting a drag from an already selected item

This means we can deselect other items immediately when performing a mouse-down
on an unselected item.

Libraries/LibGUI/GAbstractView.cpp

@@ -191,10 +191,9 @@ void GAbstractView::mousedown_event(GMouseEvent& event)
  m_selection.clear();
  } else if (event.modifiers() & Mod_Ctrl) {
  m_selection.toggle(index);
- } else if (event.button() == GMouseButton::Left && !m_model->drag_data_type().is_null()) {
+ } else if (event.button() == GMouseButton::Left && m_selection.contains(index) && !m_model->drag_data_type().is_null()) {
  // We might be starting a drag, so don't throw away other selected items yet.
  m_might_drag = true;
- m_selection.add(index);
  } else {
  m_selection.set(index);
  }