Merge file-utils.js into utils.js for cZ

All files that pull in file-utils.js also pull in utils.js, so it makes sense to merge into a single file. Whilst here, do a bit of tidying of the imported code.

[Approval Request Comment]
Regression caused by (bug #): n/a
User impact if declined: none
Testing completed (on m-c, etc.): 2.53.22pre
Risk to taking this patch (and alternatives if risky): low
String changes made by this patch: none

Comment on attachment 9480665 [details] [diff] [review]
1962242-irc-merge-utils-fileutils-25322.patch

LGTM. filepicker.show was removed in Bug 1387800 so a followup will be needed later for central. Bug 1341211 removed all other uses from c-c.

Pushed by frgrahl@gmx.net:
https://hg.mozilla.org/comm-central/rev/ef951dfed902
Merge file-utils.js into utils.js for cZ. r=frg DONTBUILD

https://gitlab.com/seamonkey-project/seamonkey-2.53-comm/-/commit/b484b58a4feb19919e080afc2cc91fdbe244e404
Merge file-utils.js into utils.js for cZ. r=frg a=frg