LibGUI: Refactor AbstractView "multi select" mode into "selection mode"

There are three possible selection modes for a GUI::AbstractView.

- NoSelection
- SingleSelection
- MultiSelection

We don't enforce these modes fully yet, this patch mostly adds them in
place of the old "multi select" flag.

Libraries/LibGUI/IconView.cpp

@@ -247,7 +247,7 @@ void IconView::mousedown_event(MouseEvent& event)
     auto adjusted_position = to_content_position(event.position());
 
     m_might_drag = false;
-    if (is_multi_select()) {
+    if (selection_mode() == SelectionMode::MultiSelection) {
         m_rubber_banding = true;
         m_rubber_band_origin = adjusted_position;
         m_rubber_band_current = adjusted_position;