Add font entry to shell section for title fonts

cairo-util has used sans font family for title fonts so far.
But sans doesn't support glyphs of some non-ascii characters.
So now let users choose a font family for titles.

To show texts correctly now using pangocairo for redering fonts.
If any single font in 'fonts' entry doesn't contain all the needed glyphs,
we use the fallback mode of pango.

This pangocairo support is enabled if configure finds pangocairo
on the build environment.
Otherwise fall back to plain cairo rendering.

The entry name is generic, 'fonts' so that other places
where draw texts can use this entry too.

Signed-off-by: Ryo Munakata <ryomnktml@gmail.com>