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.
Use a font version number to avoid rebuilding fonts all the time

Fonts used Wings's version number to ensure that they had the correct
format. That caused the font files to be rebuilt all the time.

Avoid rebuilding by introducing a new version number for fonts.