fleshed out view modes for project panel

app/widget/projectexplorer/projectexplorertreeview.cpp

@@ -1,3 +1,23 @@
+/***
+
+  Olive - Non-Linear Video Editor
+  Copyright (C) 2019 Olive Team
+
+  This program is free software: you can redistribute it and/or modify
+  it under the terms of the GNU General Public License as published by
+  the Free Software Foundation, either version 3 of the License, or
+  (at your option) any later version.
+
+  This program is distributed in the hope that it will be useful,
+  but WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+  GNU General Public License for more details.
+
+  You should have received a copy of the GNU General Public License
+  along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+***/
+
 #include "projectexplorertreeview.h"
 
 #include <QMouseEvent>
@@ -5,6 +25,8 @@
 ProjectExplorerTreeView::ProjectExplorerTreeView(QWidget *parent) :
   QTreeView(parent)
 {
+  // Set selection mode (allows multiple item selection)
+  setSelectionMode(QAbstractItemView::ExtendedSelection);
 }
 
 void ProjectExplorerTreeView::mouseDoubleClickEvent(QMouseEvent *event)