build: Clean up Grunt config and add missing csscomb run

Follows-up 953c46c4cdaf89d79c5.

* Fix csscomb configuration to include stylesheets not currently
  being cleaned up:
  - oojs-ui-mediawiki*.css
  - oojs-ui-*.svg.css
  - oojs-ui-*.svg.rtl.css

* Simplified csscomb conf using Grunt file expand pattern.

* Phase out extra 'dist-temp' directory in repository root.
  Handle inside 'dist/' instead.

* Trim duplicate slashes in 'copy' task
  Running "copy:lessTemp"
  - Reading src/styles/Dialog.less...OK
  - Writing dist/tmp//styles/Dialog.less...OK
  Running "copy:svg" (copy) task
  - Reading dist/tmp/themes/apex/images/icons/add.svg...OK
  - Writing dist//themes/apex/images/icons/add.svg...OK

* Rename colorize-svg task config 'files.src' to 'srcDir'.

  The 'files' config is reserved in Grunt for a specific kind of
  value (string pattern, array of strings patterns or array of
  descriptor objects). Most of our file tasks use this pattern
  (jshint, jscs, less, uglify, svg2png, etc.).

  However colorize-svg, as it is, does not use this pattern. It uses
  a plain object with a 'src' and 'dest' directory path.
  The 'task.files' iterator is ignored by 'colorize-svg' as it
  accesses the unprocessed 'task.data.files' directly.
  In '--verbose' mode, Grunt shows a notice for "File: [no files]"
  indicating the pattern is invalid and didn't match any files.

  Either it needs to be changed to an array of objects, and match
  files (not directories), or stop using the reserved property (so
  that Grunt doesn't waste time processing the invalid values, only
  to be ignored).

  http://gruntjs.com/api/inside-tasks#inside-multi-tasks
  http://gruntjs.com/configuring-tasks#options

* Simplify fileExists task (one run instead of four separate).

* Simplify file task configuration where the 'files' array contains
  a single object (pass it as the object directly).
  Also remove redundant 'cwd' properties.

* Fix incorrect documentation of VariantList#getVariant.

* Fix copy/paste documentation of Destination#getStylesheetPath.

Change-Id: Iadc1d2bb5cd1dfb00f71e1aa7deae654d5f5ad27
Merge "build: Clean up Grunt config and add missing csscomb run"