Use new OS.File.read()

[David Rajchenbach-Teller]

In ABP for Firefox, readFromFile could be simplified.
Recent versions of Gecko offer

 OS.File.read(path, { encoding: "utf-8" });

which performs the read and decoding entirely off the main thread.

I'll try and work on a patch in the next few days if I have a little time.

[trev]

I'm somewhat undecided on the priority here. The documentation on OS.File.read() hasn't been updated yet but it seems that the relevant bug here is ​https://bugzilla.mozilla.org/show_bug.cgi?id=961665 which means that this feature was introduced in Firefox 30. So we can either wait until Firefox 31 becomes the new ESR release (October) or we can implement this in a backwards compatible way (this might eat away most of the simplification achieved).

[trev]

Mass-closing all bugs in Adblock Plus for Firefox module, the codebase of Adblock Plus 3.0 belongs into Platform and User-Interface modules. Old bugs are unlikely to still apply.