Set explicit source file encoding in parent pom.xml

Fixes Maven warnings about using the platform default encoding.

org.eventb.texteditor.parent/pom.xml

@@ -9,6 +9,9 @@
 	<packaging>pom</packaging>
 
 	<!-- this is the parent POM from which all modules inherit common settings -->
+	<properties>
+		<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
+	</properties>
 
 	<repositories>
 		<!-- configure p2 repository to resolve against -->