* src/style.c: added the default italic font as helvetica-oblique.

* src/style.h: added declaration for new global variable
  gnumeric_default_bold_font

* src/gnumeric-util.c: enhanced the robusteness of the function
  font_get_italic_name. But it's not at a 100%, though.

* plugins/excel/ms-excel.c: slight modification to the error
  tracking mechanism (font stuff in ms_excel_set_cell_font).