[BUG #7988] more refactoring and enhancements concerning the SCSS compilation

mobile skeleton:
* remove minify jobs
* add mock content for "custom.css" which is shown only when app is created from master and no source/build version is created once yet.
* "styles.css" is gone (add own rules to "_styles.scss")

apps (mobileshowcase, playground):
* remove minify jobs
* mobileshowcase: "styles.css" is now "_app.scss"
* add .gitignore (to not bother with recreated CSS files)
* run 'compile-scss' job along with the following jobs:
   * source
   * source-hybrid
   * source-all
   * build