GtkFontChooserWidget: Use a better display name for the font

Instead of reconstructing a display name from the
PangoFontDescription, use the font family and face
objects, which have the original font. This lets us
get the names of fonts like Noto Sans CJK DemiLight
right, which would be shown as Noto Sans CJK SemiLight
when mangled via PangoFontDescription, since Pango
treats 'DemiLight' as an alias for the SemiLight weight.

https://bugzilla.gnome.org/show_bug.cgi?id=733832
GtkFontButton: Use the same font name as the font chooser

The font chooser constructs the display name for the font from
the family and face names. Do the same in the font button, so
we don't end up calling the same font by different names, which
would be confusing.
GtkFontChooserWidget: Allow fractional font sizes

With this change, we let users enter up to 2 decimals in the entry,
to specify font sizes exactly. The slider still rounds to whole
numbers.

https://bugzilla.gnome.org/show_bug.cgi?id=665711
GtkFontButton: Limit the decimals shown for size

It doesn't make sense to show a font size with 6 digits. Use
the same formatting we use in the font chooser widget now.

https://bugzilla.gnome.org/show_bug.cgi?id=724996
GtkFontButton: Avoid criticals

The font_family and font_face members may be NULL, and we
shouldn't cause critical warnings in that case.
Updated POTFILES.in
GtkFontChooser: Prevent font fallback in the preview

We don't want to preview the fallback fonts, after all.
https://bugzilla.gnome.org/show_bug.cgi?id=570414
Add an example for a hig-style toolbar
Replace uses of g_memmove() by memmove()

g_memmove() is deprecated, it is a simple macro that just calls
memmove() with the same parameters.

Reviewed by Paolo Borelli on IRC.