Improve [convert_js] metadata section syntax in adblockpluschrome build scripts

[kzar]

Background

The build scripts for adblockpluschrome support a convert_js metdata section in order to transpile ES6 JavaScript code at buildtime. It works pretty well but the syntax is cumbersome, causing us problems when you want to add an input file to the list using the += syntax introduced with #2711.

What to change

With the current syntax, an example use of convert_js looks like this:

[convert_js]
lib/adblockplus.js = adblockpluscore/lib/events.js
  lib/prefs.js
  lib/utils.js
  ...
  --arg module=true source_repo=https://hg.adblockplus.org/adblockpluscore/

Update the buildscripts in ​buildtools so that the same example would instead look like this:

[convert_js]
lib/adblockplus.js = adblockpluscore/lib/events.js
  lib/prefs.js
  lib/utils.js
  ...
lib/adblockplus.js[module] = true 
lib/adblockplus.js[source_repo] = https://hg.adblockplus.org/adblockpluscore/

Also update the buildtools dependency in adblockpluschrome, along with any metadata files where the convert_js section is used.

Notes

• The arguments module and source_repo are just examples, they should not be hardcoded, rather any arguments should be supported with the syntax.
• The +=/-= syntax used with metadata inheritance must work for the convert_js options. For example we might want to do lib/adblockplus.js += some/safari/specific/file.js or even lib/adblockplus.js:someOption += something.

[sebastian]

Jon and Vasily, does any of you'd like to work on this?

[sebastian]

Apparently, ConfigParser is considering colons (:) as separators just like = characters. So I had to change the syntax.

[abpbot]

A commit referencing this issue has landed:
​Issue 4047 - Improved configuration of converted JS files