Projects mode: Make plus icon dark and get rid of branch indicators

The PLUS icon was actually PLUS_TOOLBAR, so name it that way and add an
actual PLUS icon.
Remove the drawing of the branch indicators from the kit tree view by
overriding the method that does it.

Change-Id: I395e5187c1738faaee3e122b4f3f359261b514d7
Reviewed-by: hjk <hjk@qt.io>

src/plugins/projectexplorer/projectwindow.cpp

@@ -451,6 +451,12 @@ public:
         setObjectName("ProjectNavigation");
         setContextMenuPolicy(Qt::CustomContextMenu);
     }
+
+    // remove branch indicators
+    void drawBranches(QPainter *, const QRect &, const QModelIndex &) const override
+    {
+        return;
+    }
 };