GUACAMOLE-2139: Inherit guacamole-client version via "revision" property (supported since Maven 3.5.0).

pom.xml

@@ -26,7 +26,7 @@
     <groupId>org.apache.guacamole</groupId>
     <artifactId>guacamole-client</artifactId>
     <packaging>pom</packaging>
-    <version>1.6.1</version>
+    <version>${revision}</version>
     <name>guacamole-client</name>
     <url>http://guacamole.apache.org/</url>
 
@@ -35,6 +35,12 @@
         <!-- Core properties -->
         <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
 
+        <!-- The Guacamole version for the entirety of guacamole-client (this is
+        referenced by this and all other pom.xml files within guacamole-client,
+        and is a special case within Maven as one of the few properties allowed
+        within the pom.xml version and intended for this purpose) -->
+        <revision>1.6.1</revision>
+
         <!-- Dependency versions -->
         <guava.version>32.1.3-jre</guava.version>
         <guice.version>5.1.0</guice.version>