Preferences setting to combine the axis trace and the value label buttons

Enabled the value label on the horizontal axis, which I accidentally removed.

Added an option (for ITER) to combine the buttons to show the value labels and axis traces. 
If property org.csstudio.swt.xygraph/combine_labels_and_traces is set to true, the buttons will be combined, if set to false, the buttons will be separated as they are now.