Make pqDoubleLineEdit not use a format specific double validator.
pqDoubleLineEdit should always accept any valid double string. It should
only change how it's shown based on the chosen format.

Now, pqDoubleLineEdit also doesn't attempt to cleanup a number entered
by the user. Thus, if user enter `1e10` for the full precision text, the
full precision text stays as so.

Changed default precision used by the widget to mixed rather than fixed.

(cherry picked from commit b02aa392)