dpll.js/index.html: varsDict doesn't need to be known by the API user

Signed-off-by: Diego Giovane Pasqualin <dpasqualin@c3sl.ufpr.br>
dpll.js: Fix variable names on node popup

We were showing the internal name of the variables instead of the symbol
that the user actually chose to represent them

Signed-off-by: Diego Giovane Pasqualin <dpasqualin@c3sl.ufpr.br>
dpll.js: Print var name given by user on the Graph nodes

We were printing the variable internal names.

Signed-off-by: Diego Giovane Pasqualin <dpasqualin@c3sl.ufpr.br>