[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/build.gradle

@@ -7,6 +7,12 @@ plugins {
     id 'signing'
 }
 
+java {
+    toolchain {
+        languageVersion.set(JavaLanguageVersion.of(8))
+    }
+}
+
 dependencies {
     testImplementation 'org.junit.jupiter:junit-jupiter-api:5.5.2'
     testImplementation 'org.junit.jupiter:junit-jupiter-engine:5.5.2'