[datlechin]

Summary

• A sortable column sized by its header now counts the sort indicator GTK keeps beside the title (16 px plus a 6 px gap), so the title and arrow are no longer clipped.
• The header width is measured in the style Adwaita draws it in, .caption-heading: bold at 82 % of the grid's font.
• Changing the interface font, the text size or the monospace font while a grid is open measures the columns the grid sized itself again. Widths the user set stay as they are. The row number column refits as well.
• The font settings reach the grid through WidgetImpl::system_setting_changed (Dpi, FontName, FontConfig), and the monospace font through AdwStyleManager notify::monospace-font-name. That handler is dropped at dispose.
• A grid that was out of its window when a font changed, or was sized before it had one, measures again when it is rooted in fonts other than the ones it measured in. Measuring reads a fresh Pango context, since a widget's own follows a change only at the next frame.

Follow-up to #3895.

Test plan

• ui::data_grid::tests::columns:
  • a_sorted_columns_header_shows_its_title_and_arrow_whole
  • a_text_size_change_measures_the_grids_columns_again
  • a_monospace_font_change_measures_the_grids_columns_again
  • a_grid_back_in_its_window_takes_a_text_size_it_missed
• CI runs the build, tests and lint. The local Linux container was removed to free disk space, so no local run and no screenshots.