[Users] GTK3 font handling

[Pierre Fortin]

1. Preferences/Display/Fonts/Folder and Message Lists:
   a) If there is no content displayed in Message List, changing font
   size does not update the row heights on Folder List; but works if
   there is content in Message List.  This issue was difficult to
   summarize due to dependencies as to what is displayed -- may not be
   immediately reproducible... 
   b) this font selector also affects Message Headers -- these should be
   controlled by the Preferences/Display/Fonts/Message font size.

2. Menus, Toolbar buttons, Preferences window, (others?):
   I run two instances of CM3 (same issue is seen in CM2) -- one matches
   the Folder/Message List and Message font sizes; the other's Menus,
   Toolbar buttons & Preferences window all display a TINY font (9). Is
   there a way to control font size on these?