* src/item-bar.c (item_bar_calc_size): Use default desktop font of
screen.

* src/workbook-control-gui.c: Add PivotTable verb.
(settings_get_font_desc): New. Get Pango font desc from string.
(cb_update_item_bar_font): New callback. Called from
gtk_container_foreach for each notebook tab to update the item bars.
(cb_desktop_font_changed): New callback. Called when desktop font
has changed.
(wbcg_get_gtk_settings): New. Gets GtkSettings - by screen if
possible.

* src/workbook-control-gui-priv.h (struct _WorkbookControlGUI):
Add font_desc.