Simplify font handling and stop using parameterized modules

The wings__font.erl module uses the experimental feature
"parameterized modules", which is scheduled to be removed in
Erlang/OTP R16.

It turns out that wings__font for historical reasons is more
flexible than it needs to be and that it is almost unused. Most
of its functionality has been taken over by wings_font_table.
By slightly tweaking wings_text and wings_font_table we can easily
eliminate the need for wing__font.

By going a step further, we can see that there is no need to keep
the code in wings_font_table in a separate module. There is not
a large amount of code, and having code split into two modules
does not make it easier to understand since there is
wings_font_table does not hide any information from wings_text.

Therefore, move all font handling code into wings_text and remove
both wings__font and wings_font_table.