Add kmscon_font type

Add kmscon_font type

A kmscon_font object is used to store the current font information. It allows to
draw any kind of UTF-8 string to the screen. Internally, it uses kmscon_glyph to
store glyph information for every character that has been drawn so redrawing it
is much faster.

Currently, we only support GLYPH_LAYOUT as caching method which is quite slow.
However, it supports any kind of input and always works. Better and faster
caching algorithms like cairo_scaled_font_t will be added later.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>