Can now add color schemes as separate json files in {dir}/themes

[AviFS]

Let me know how it looks! I'll be more than happy to refactor it if the project style is different.

I have @abrudz's blessing here to implement the idea. Of course, whether or not this implementation hits the mark is TBD.

[AviFS]

As hacky as it is, if we doubt the safeness, we can just wrap the piece that I committed in a big try/catch:

// Add JSON color schemes from {path}/themes/ to SCMS
// The {path} is the same as in {path}/prefs.json
try {
    let SCMScustom;
    const fs = nodeRequire('fs');
    const el = nodeRequire('electron').remote;
    const dirPath = `${el.app.getPath('userData')}/themes/`;
    fs.readdirSync(dirPath).forEach(file => {
      // Get JSON file contents & validate files
      let fileContents; let fileJSON;
      try { fileContents = fs.readFileSync(dirPath+file, {encoding: "utf8"}); } 
      catch(e) { console.error(`Couldn't open themes/${file}.`); return; }
      try { fileJSON = JSON.parse(fileContents); }
      catch (e) { console.error(`themes/${file} could not be parsed as JSON.`); return; }

      // If files validated, then add scheme to SCMScustom
      if (fileJSON.name && fileJSON.theme && fileJSON.styles) { SCMScustom.push(decScm(fileJSON)); }
      else { console.error(`themes/${file} missing one or more of 'name', 'theme' or 'styles'.`) }
  })
}
catch (e) { console.error("See PR #749: " + e); }
// If everything worked, we can safely modify SCMS
finally { SCMS.push(SCMScustom); }

It looks silly, but worst-case scenario, it would just fall back to the behavior implemented before, while leaving a trace. The built-in themes would stay put in SCMS, and the prefs.json are already loaded into D.prefs.colourSchemes. It just wouldn't add the ones in themes/. No variables outside of the scope get modified, unless it runs successfully, in which case all custom themes are added to SCMS.

NOTE: I'm still working on the syntax of this version. Should be finalized in a few minutes.

[abrudz]

I'm closing this for now, pending conclusions from #751