Make the toolbox window not resizable.

plugins/Toolbox/resources/qml/Toolbox.qml

@@ -16,9 +16,10 @@ Window
 
     width: 720 * screenScaleFactor
     height: 640 * screenScaleFactor
-    minimumWidth: 720 * screenScaleFactor
-    maximumWidth: 720 * screenScaleFactor
-    minimumHeight: 350 * screenScaleFactor
+    minimumWidth: width
+    maximumWidth: minimumWidth
+    minimumHeight: height
+    maximumHeight: minimumHeight
     color: UM.Theme.getColor("sidebar")
     UM.I18nCatalog
     {