Change Skeleton folder name to MainWindow.

This folder is intended to store the files that construct the main
window. All the parts. The rest of the components are stored in more
specific folders.

Contributes to CURA-5784.

resources/qml/Cura.qml

@@ -12,7 +12,7 @@ import Cura 1.1 as Cura
 
 import "Dialogs"
 import "Menus"
-import "Skeleton"
+import "MainWindow"
 
 UM.MainWindow
 {