Add support for downloading nick icon themes from store.kde.org
I discovered there is a category of Konversation nick icon themes on store.kde.org at https://store.kde.org/browse/cat/405 While the store items are dating back to 2011-2005, they still work (except for the one misplaced emoticon set).
To make those alternatives also discoverable to anyone else, this patch adds KNewStuff usage to the theme selection UI, by adding a button "Get New Themes...":
Which then opens the dialog to install or uninstall nick icon themes as provided on store.kde.org:
