Add JavaScript Event Object handling

Summary:
This adds a new data object "Event" that can be used
to carry information in and out of JavaScript execution
contexts. The Event Object is defined in the Adobe JavaScript
scripting reference.

The implementation now adds handling for the FieldCalculate
Event. It should be extensible enough so that in the future
more events could be supported.

Reviewers: #okular

Subscribers: aacid

Tags: #okular

Maniphest Tasks: T7805

Differential Revision: https://phabricator.kde.org/D10073