Upgrade to Eclipse 2018-09

[lwrage]

Includes upgrade to Xtext 2.15.

[lwrage]

waiting for eclipse-xsemantics/xsemantics#112

[lwrage]

At least on Linux diagram fonts look significantly smaller in 2018-09 on my laptop. No idea what has changed. Need to test on Mac and Windows.
Font sizes are defined in enumeration org.osate.ge.internal.ui.properties.AppearancePropertySection.FontSize as Default(null), Small(8.0), Medium(10.0), Large(16.0), ExtraLarge(20.0);
org.osate.ge.graphics.Style.DEFAULT defines the default font size as 10.0.