Consider renaming leftover style-context apis
The only non-deprecated gtk_style_context apis are
- gtk_style_context_add_provider_for_display
- gtk_style_context_remove_provider_for_display
We should rename this to something that fit better. Suggestions could be:
- gtk_add_css_style_sheet
- gtk_remove_css_style_sheet
After renaming GtkCssProvider to GtkCssStyleSheet.