Build: Set javac release property (ticket #2775)

Ant version 1.9.8 or higher now required
Drop support for Xenial package build
Fix up BOB build configuration
Fix i2psnark standalone build

apps/desktopgui/build.xml

@@ -8,6 +8,7 @@
 	<property name="javadoc" value="javadoc"/>
 	<property name="javac.compilerargs" value=""/>
 	<property name="javac.version" value="1.8" />
+	<property name="javac.release" value="8" />
 	<property name="require.gettext" value="true" />
 
     <condition property="no.bundle">
@@ -27,6 +28,7 @@
 
         <target name="compile" depends="init">
             <javac debug="true" deprecation="on" source="${javac.version}" target="${javac.version}" 
+                       release="${javac.release}"
                        includeAntRuntime="false"
                        encoding="UTF-8"
                        srcdir="${src}" destdir="${build}">
@@ -52,6 +54,7 @@
                 <arg value="./bundle-messages.sh" />
             </exec>
             <javac source="${javac.version}" target="${javac.version}" 
+                   release="${javac.release}"
                    includeAntRuntime="false"
                    encoding="UTF-8"
                    srcdir="${build}/messages-src" destdir="${build}">