Fix fonts in context menus

Context menus inherit their style from the widget they are attached
to. This can have unexpected effects in particular for font, as can
be seen in gedit or gnome-calculator. Prevent this by resetting
the font of context-menus to the initial value.

gtk/resources/theme/Adwaita/_common.scss

@@ -2825,3 +2825,8 @@ GtkVolumeButton.button { padding: 8px; }
   &:backdrop { color: if($variant=='light', $backdrop_base_color,
                                             $backdrop_fg_color); }
 }
+
+/* Decouple the font of context menus from their entry/textview */
+.context-menu {
+  font: initial;
+}