Use prefs.js to get installed torbrowser version

[nickjwhite]

Browser/TorBrowser/Docs/sources/versions is no longer provided by
TorBrowser, so get the current version string from prefs.js instead.

This fixes issue #306.

Described in https://trac.torproject.org/projects/tor/ticket/25012#comment:8 as "a more robust kludge" than reading Changelog.txt :p

[aagbsn]

From IRC (OFTC, #tor-dev) discussion:

< aagbsn> is parsing prefs.js the ugly workaround? it looks like someone has a PR on github.com/micaflee/torbrowser-launcher/pull/308
< aagbsn> er that link is bad. sorry. from another device. should be github.com/micahflee/torbrowser-launcher/pull/308
<+sysrqb> aagbsn: prefs.js is a temp file generated when firefox loads based on the current browser prefs 
<+sysrqb> it's not really intended for specifying a version number,
< aagbsn> oh catch 22
< aagbsn> if tbb hasn't launched, it doesn't exist?
<+sysrqb> hmm, let me check a fresh tar.gz
<+sysrqb> yeah, nope
<+sysrqb> this is what the header of prefs.js says, btw:
<+sysrqb>  *
<+sysrqb>  * If you make changes to this file while the application is running,
<+sysrqb>  * the changes will be overwritten when the application exits.
<+sysrqb>  *
<+sysrqb>  * To make a manual change to preferences, you can visit the URL about:config
<+sysrqb>  */
< aagbsn> bummer scene 
< boklm> so the patch reading the version from the ChangeLog file looks better (although it can still be broken easily in a future version, but 
         should fix the issue for now)
<+sysrqb> okay, but it is saved across loads 
<+sysrqb> (prefs.js)
<+sysrqb> so torbrowser-launcher may get a stale vers from prefs.js, sbut it should work as long as TB was previously launched
<+sysrqb> the ChangeLog seems slightly better, because it's a fixed file
<+sysrqb> but there aren't any guarantees about the format of that, either
<+sysrqb> so *shrug*
<+sysrqb> i guess they'll do whatever is the easiest fix
< aagbsn> do you mind if I paste the above comments on the github PR?
< boklm> with the prefs.js patch, torlauncher will try to load this file before running torbrowser for the first time, so it won't work
< intrigeri> long term, I guess the TB build process should ideally create a metadata file about itself. similar to /etc/os-release, what Tails 
             has in /etc/amnesia/version, etc.
<+sysrqb> aagbsn: nope, go for it
<+sysrqb> intrigeri: yeah (i'm not sure if you saw yawning created #25020 )
-zwiebelbot:#tor-dev- tor#25020: There should be an easy way to detect the version of an installed bundle. - [new] - 
    https://bugs.torproject.org/25020
< aagbsn> boklm, intrigeri ok also? 
< intrigeri> aagbsn: yep.

[nickjwhite]

Thanks @aagbsn for posting that, I should have tested more thoroughly; I presumed that the default prefs.js was part of the shipped tarball. I'll close this pull request in favour of #307.