fontchooser: Don't cause "row-changed" signal in cell data func

fontchooser: Don't cause "row-changed" signal in cell data func

The font chooser delays creating the font description from the font face
as long as possible (it's slow). Because we use fixed height mode, we
only have to create font descriptions for rows we are actually going to
show.

This was achieved by looking at the font description column and if it
was NULL, we created a font description and gtk_list_stiore_set() it.
Unfortunately this caused a "row-changed" signal to be emitted and this
emission could happen during the cell data func.
And that caused infinite loops with accessibility when you were unlucky.

This change replaces the NULL font description with an empty one and
instead of setting the correct font description, we
pango_font_description_merge() it in. This way, the list store doesn't
change and no signals are emitted.

https://bugzilla.redhat.com/show_bug.cgi?id=1197267