fontchooser: Rewrite size changes

fontchooser: Rewrite size changes

They now go through gtk_font_chooser_widget_take_font_desc(). The end
goal is to make all changes go through this function, so that all
updates that are happening are easy to track.

Another change is that the code now merges the font description instead
of just using the new one. This avoids weird situations when people set
the font "Bold" for example, which has neither a size nor a font family.