#272 JavaScript code generation step 1: add export menu option

• Added JavaScript export option to export menu.
• Added empty search/replace template (to be filled in later).
• Created empty stub methods for most implementations (to be filled in later).

• Export feature works without problems/errors, but currently only generates an empty .html file.

For discussion, see: riklubking/escet#27

Addresses #272 (closed)