The welcome dialog is now a rectangle instead of a dialog

The whole window approach was just causing to much cross platform issues, so
I converted it into a rectangle and put a mouseArea that eats all the events in the
greyOut background

CURA-6057

resources/qml/WelcomePages/WelcomeDialog.qml

@@ -13,18 +13,16 @@ import Cura 1.1 as Cura
 //
 // This is a no-frame dialog that shows the welcome process.
 //
-Window
+Item
 {
     UM.I18nCatalog { id: catalog; name: "cura" }
 
     id: dialog
-    title: catalog.i18nc("@title", "Welcome to Ultimaker Cura")
-    modality: Qt.ApplicationModal
-    flags: Qt.Dialog | Qt.FramelessWindowHint
+
+    anchors.centerIn: parent
 
     width: 580 * screenScaleFactor
     height: 600 * screenScaleFactor
-    color: "transparent"
 
     property int shadowOffset: 1 * screenScaleFactor
 

resources/qml/WelcomePages/WizardPanel.qml

@@ -25,6 +25,8 @@ Item
     property var progressValue: model == null ? 0 : model.currentProgress
     property string pageUrl: currentItem == null ? "" : currentItem.page_url
 
+    property alias backgroundColor: panelBackground.color
+
     signal showNextPage()
     signal showPreviousPage()
     signal goToPage(string page_id)  // Go to a specific page by the given page_id.
@@ -41,7 +43,7 @@ Item
         id: panelBackground
         anchors.fill: parent
         radius: UM.Theme.getSize("default_radius").width
-
+        color: UM.Theme.getColor("main_background")
         Cura.ProgressBar
         {
             id: progressBar