WIP: Render im span as selection
I attempted to have TextArea apply a font style via a custom PangoAttr, private to GTK. The attribute works when it's created directly in the renderer, but not when it's passed from the IM. It gets stripped from the attributes passed to the renderer.
I don't know how to provide a reduced test case, since GtkTextView does't seem to provide a way to directly appy PangoAttrs like imcontext does.