Fix opening more than one get new applets windows

A new instance WidgetExplorer is created everytime we open get new
applets window. Each and every instance of WidgetExplorer has its own
newStuffDialog. This means that downloadWidgets method will always
create new instance of QtQuickDialogWrapper due to the default value of
newStuffDialog is null.

Change newStuffDialog to static so that it will be shared to every
instances of WidgetExplorer. This prevents another get new applets
window to be opened.

BUG: 412765