* glom/mode_data/datawidget/datawidget.cc: signal_style_changed was removed
though it might come back later.
* glom/mode_data/datawidget/entry.cc: Use override_color() instead of
modify_text().
* glom/mode_data/placeholder-glom.cc: Use set_source_rgba() instead of
set_source_color(). Use get_style_context() instead of get_style().