- fix splash screen to show black background on linux and windows

src/main/kotlin/mediathek/SplashScreen.kt

@@ -4,6 +4,7 @@ import mediathek.config.Konstanten
 import mediathek.tool.TimerPool.timerPool
 import mediathek.tool.UIProgressState
 import org.apache.commons.lang3.SystemUtils
+import org.jdesktop.swingx.StackLayout
 import java.awt.Color
 import java.awt.Cursor
 import java.awt.Dimension
@@ -20,6 +21,9 @@ class SplashScreen : JWindow() {
     private val imageLabel = JLabel()
     private val progressBar = JProgressBar()
     private val statusLabel = JLabel()
+    private val bgPanel = JPanel()
+    private val contentPane_top = JPanel()
+
 
     init {
         initComponents()
@@ -64,6 +68,7 @@ class SplashScreen : JWindow() {
      * @param percentComplete The new percentage.
      */
     private fun updateStatus(statusText: String?, percentComplete: Int) {
+        bgPanel.paintImmediately(0,0, width, height)
         appTitleLabel.paintImmediately(0, 0, appTitleLabel.width, appTitleLabel.height)
         imageLabel.paintImmediately(0, 0, imageLabel.width, imageLabel.height)
         versionLabel.paintImmediately(0, 0, versionLabel.width, versionLabel.height)
@@ -100,8 +105,16 @@ class SplashScreen : JWindow() {
         statusLabel.foreground = Color.white
         statusLabel.background = Color.black
         statusLabel.isOpaque = true
-        val contentPaneLayout = GroupLayout(contentPane)
-        contentPane.layout = contentPaneLayout
+        val stackPanel = JPanel(StackLayout())
+        contentPane = stackPanel
+
+        bgPanel.background = Color.black
+        contentPane_top.background = Color.black
+        stackPanel.add(bgPanel, StackLayout.BOTTOM)
+        stackPanel.add(contentPane_top, StackLayout.TOP)
+
+        val contentPaneLayout = GroupLayout(contentPane_top)
+        contentPane_top.layout = contentPaneLayout
         contentPaneLayout.setHorizontalGroup(
             contentPaneLayout.createParallelGroup()
                 .addGroup(