Cleanup unused stuff and fix detekt issues

src/main/kotlin/com/marvinelsen/willow/Main.kt

@@ -2,4 +2,4 @@ package com.marvinelsen.willow
 
 fun main(args: Array<String>) {
     actualMain(args)
-}
+}

src/main/kotlin/com/marvinelsen/willow/ui/util/Concurrency.kt

@@ -1,7 +0,0 @@
-package com.marvinelsen.willow.ui.util
-
-import javafx.concurrent.Task
-
-fun <T> task(block: () -> T) = object : Task<T>() {
-    override fun call() = block()
-}