It's now possible to edit widget properties inline in the properties window.
We're currently relying on the basic GVariant conversion functions to do
all the "parsing" but that's not gonna be good enough.
This is essentially a combo widget containing a single-line GTextEditor
and two buttons for increment and decrement. The GTextEditor::on_change
callback is hooked to prevent non-numeric input but it's not entirely
perfect since that callback is asynchronous. This will work until we have
some more sophisticated input validation mechanism though.
