ProjectExplorer: Allow users to quickly expand the project tree

... by adding an "Expand All" action to accompany the existing "Collapse
All" one.

Fixes: QTCREATORBUG-17243
Change-Id: I84e185562ee4c62ede7d0dc40f0d296020c4f0ea
Reviewed-by: hjk <hjk@qt.io>

src/plugins/projectexplorer/projecttreewidget.cpp

@@ -413,6 +413,11 @@ void ProjectTreeWidget::collapseAll()
     m_view->collapseAll();
 }
 
+void ProjectTreeWidget::expandAll()
+{
+    m_view->expandAll();
+}
+
 void ProjectTreeWidget::editCurrentItem()
 {
     m_delayedRename.clear();