[Build] Add shadowJar plugin to collect all deps into one jar

resolves #52

imgui-app/build.gradle

@@ -1,5 +1,6 @@
 plugins {
     id 'java-library'
+    id 'com.github.johnrengelman.shadow' version '7.0.0'
     id 'checkstyle'
     id 'maven-publish'
     id 'signing'
@@ -40,5 +41,9 @@ jar {
     }
 }
 
+shadowJar {
+    with jar
+}
+
 apply from: "$rootDir/publish.gradle"
 configurePublishing('imgui-java-app', 'Application wrapper for Dear ImGui')