Port welcome screen to Qt Quick 2

This ports the welcome screen to use QtQuick 2.0.
I have disabled the plugin on Qt4 builds.

Change-Id: Ia921d0747c8f7d4441c88fc9fb77b822496091f4
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>

src/plugins/plugins.qbs

@@ -2,6 +2,7 @@ import qbs
 
 Project {
     name: "Plugins"
+
     references: [
         "analyzerbase/analyzerbase.qbs",
         "android/android.qbs",
@@ -56,6 +57,11 @@ Project {
         "updateinfo/updateinfo.qbs",
         "valgrind/valgrind.qbs",
         "vcsbase/vcsbase.qbs",
-        "welcome/welcome.qbs"
     ]
+
+    Group {
+        condition: qtcore.versionMajor >= 5
+        references: [ "welcome/welcome.qbs" ]
+    }
+
 }