[API] Fix dependency tree issue for imgui-java-app

resolve #75
resolve #127

Removed shadowing for `imgui-java-app.jar` which is published to Maven Central.
Shadow Jar now has a prefix `-all` - for those one who prefer to use raw jars instead dependencies in maven/gradle.

imgui-binding-natives/build.gradle

@@ -4,6 +4,12 @@ plugins {
     id 'signing'
 }
 
+java {
+    toolchain {
+        languageVersion.set(JavaLanguageVersion.of(8))
+    }
+}
+
 def packageName = 'imgui-java-natives-linux'
 def packageDesc = 'Native binaries for imgui-java binding for Linux'
 def moduleName = 'imgui.natives.linux'