Fixes #1541, relative path require fail on Ubuntu 20.04

[blaze33]

cf . #1541

For some (still unexplained) reason, in bootstrap.js, relative imports ie. require("./some-relative-path") fail.

This PR fixes the issue in two ways:

1. Avoiding the import altogether: require('../../package.json') is replaced by nw.App.manifest.
2. Rewriting relative path to absolute ones.

[blaze33]

As explained in #1541 (comment) the relative ../package.json import only fails when special characters are in the import path. Not sure how to handle this case apart from moving PCT files to a folder path with no special characters.

Considering you say #1784 conflicts with this PR, I'm okay with you closing it.