Change pages organization

- UI (previously MainUI) is back to being the only component loaded
  as Window's child

- UI has the background image previously only for the SignInPage

- If there are no accounts, the UI Loader's initialItem is the
  SignInPage

- The SidePane becomes visible when there's >=1 account connected

harmonyqml/components/Window.qml

@@ -9,8 +9,7 @@ ApplicationWindow {
 
     Loader {
         anchors.fill: parent
-        source: Backend.clientManager.clientCount < 1 ?
-                "pages/LoginPage/LoginPage.qml" : "pages/MainUI.qml"
+        source: "UI.qml"
         objectName: "UILoader"
     }
 }