* Changing styles no longer affects existing windows

* Changing styles no longer affects existing windows
* glk_style_hint_clear() now works as intended
* Support for changing stylehints on textgrids

There are now four sets of tags stored inside two style_sets:
default->text_grid
default->text_buffer
current->text_grid
current->text_buffer

The default styles get loaded from CSS (hardcoded if not present in CSS)
The current styles get changed all the time and copied to a new window upon
creation.