[BUG #7988] remove 'flat.css' and 'indigo.css' and create them via Sass during publish instead
[BUG #7988] don't commit built qx.Mobile theme files (SCSS -> CSS) to repository

* rm {flat,indigo}.css
* add .gitignore (with 'flat.css' and 'indigo.css')
* add 3 new make targets in 'Makefile.release'
   * resources-framework-check
   * resources-framework-build-css
   * resources-framework-build
* add two new generator (config) jobs
   * compile-scss (in 'config.tmpl.json' for mobile)
   * compile-framework-scss (in 'base.json')
* add instructions to 'readme.txt' if app was created without built 'indigo.css'
* support 'command-not-found' subproperty (message string) for generator config key 'shell' (which runs shell jobs)
* run 'compile-scss' job along with the following mobile jobs:
   * source
   * source-hybrid
   * source-all
   * build