This is our first client of the new JSON GUI declaration thingy.
The skeleton of the TextEditor app GUI is now declared separately from
the C++ logic, and we use the Core::Object::name() of widgets to locate
them once they have been instantiated by the GUI builder.
So far, clicking on a link from the Markdown/HTML preview Web::PageView did
nothing - now we pass that link to Desktop::Launcher, which will then
open it in Browser, FileManager, another TextEditor instance etc.
