Fixed problem of player looking for CSS file in a hard-coded location.

Fixed problem of player looking for CSS file in a hard-coded location.
Dug up a lot of other problems doing that:
* Moved style initialization to the constructor of ChimaraGlk
* Now styles are always initialized, so we don't have to keep track of
  whether they are or not
* Moved style initialization and CSS parsing out of the Glk thread, into the
  main GTK thread, since it's controlled by the widget API not the Glk API
* Removed the 'default-font-description' and 'monospace-font-description'
  properties and their associated API
* Added API chimara_glk_set_css_to_default() [not implemented yet],
  chimara_glk_set_css_from_file(), chimara_glk_set_css_from_string()