Add print support

[Lesik]

The code was tested by me on an Android 7.1 device and works.

Some caveats:

• Due to the way that CopperPDF displays PDFs, only the current page can be printed, not the whole document.
• If the user selects print before the document fully loaded, the resulting print job may be incomplete. I couldn't figure out a way to get a callback from PDF.js when the document has successfully loaded.

[Lesik]

[paride]

Thanks! Will test (and merge) in a couple of days.

[Tommy-Geenexus]

@Lesik
Take a look at https://github.com/Applications-Development/platform_packages_apps_PdfViewer/commit/2017c867f69d038872bd0ee4acb3d9525cf5ddf2.

You can modify viewer.js and add a javascript interface acting as a callback.
Disable the print menu item initially, and enable it once the callback gets invoked,
then disable again once a new PDF gets selected.

[Lesik]

@Tommy-Geenexus Thanks, but are we allowed to take inspiration from your code? It is under a non-free license, so I deliberately didn't look at it, even if it was tempting...

[Tommy-Geenexus]

@Lesik Sure. I originally planned to get my fork on FDroid, but i forgot about the restrictive license.
Eventually i'll try to pr my changes to this fork, because they implement basic necessary features.

[paride]

I'm very sorry for the delay in merging this. I just finished (temporarily) relocating in another country, and I've been under pressure with other things. Will come back to this asap.