[BUG #7810] renamed 'grunt_qx' to 'common', 'qx' to 'generator_config'

- to have a cleaner separation between 'old' and 'new' settings
[BUG #7810] adapted website's Gruntfile.js
[BUG #7810] add an interrupt handler to clean up temp. config file

- for generator shell exits
[BUG #7810] adapted website widget browser's Gruntfile.js
[BUG #7810] centralize QOOXDOO_VERSION in grunt/config/application.js

- for the time being, until we have a 'getQxVersion' Grunt task
[BUG #7810] adapted framework's Gruntfile.js