Add support for form text formatting

Summary:
With formatting there is an internal value, which represents
the true value of a field additionaly to the normal,
visible, text.

For fields which have formatting rules these might differ
and for calculations the internal value is used. The behavior
to format on focus in / focus out events is similar to
that of Acrobat reader.

Test Plan: Needs unit test

Reviewers: aacid

Subscribers: okular-devel

Tags: #okular

Maniphest Tasks: T8886

Differential Revision: https://phabricator.kde.org/D13171