Move the Preferences initialization/fetching code to the top of PDFViewerApplication.initialize, and add a enhanceTextSelection preference to the viewer

[Snuffleupagus]

Please refer to the individual commit messages.

This PR should make it easier to switch the text-selection modes, e.g. for someone working on the tasks in issue #7584. It should also simplify things for third-party implementers of the default viewer, since this allows you to choose text-selection mode without having to edit the code.

/cc @yurydelendik You suggested on IRC that we move the Preferences initialization, so I'm submitting this for feedback.

Edit: Probably easier reviewing with https://github.com/mozilla/pdf.js/pull/7586/files?w=1.

[Snuffleupagus]

/botio-linux preview

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Published

• Viewer: http://107.21.233.14:8877/38926ca0047cabb/web/viewer.html
• Extension: http://107.21.233.14:8877/38926ca0047cabb/extensions/firefox/pdf.js.xpi

[timvandermeij]

Thanks!

[Snuffleupagus]

Unfortunately it seems that this actually caused issues, e.g. with the localized event in the Firefox addon firing before the handler is ready, since it seems that we have a bunch of code that relies on certain PDFViewerApplication properties being synchronously available.
I cannot see an immediate fix for these issues, so I'm afraid that we have to back this out for now since finding a proper solution might be trickier than I first imaged.

My apologies for not testing this thoroughly before submitting the PR!

[timvandermeij]

I'll check to see how we can revert the commit.