src/Tools/GraphBrowser/etc/build.props

clarified build_props: empty module means no build;
clarified signature;
clarified errors;

title = graph browser
module = $ISABELLE_HOME/lib/classes/isabelle_graphbrowser.jar
javac_options = -source 7 -target 7
sources = \
  awt/Border.java \
  awt/MessageDialog.java \
  awt/TextFrame.java \
  graphbrowser/AWTFontMetrics.java \
  graphbrowser/AbstractFontMetrics.java \
  graphbrowser/Box.java \
  graphbrowser/Console.java \
  graphbrowser/DefaultFontMetrics.java \
  graphbrowser/Directory.java \
  graphbrowser/DummyVertex.java \
  graphbrowser/Graph.java \
  graphbrowser/GraphBrowser.java \
  graphbrowser/GraphBrowserFrame.java \
  graphbrowser/GraphView.java \
  graphbrowser/NormalVertex.java \
  graphbrowser/ParseError.java \
  graphbrowser/Region.java \
  graphbrowser/Spline.java \
  graphbrowser/TreeBrowser.java \
  graphbrowser/TreeNode.java \
  graphbrowser/Vertex.java