Added support for theme in config.xml

Implements issue # blackberry/BB10-Webworks-Packager#206

Set theme for the application controls base one of the values: 'dark', 'default', 'bright', 'inherit'.
If no value set or the value spelled wrong, 'default' would be used.

__Note__: This feature is NOT for R9E.

[More info about the feature](https://github.com/blackberry/BB10-Webworks-Packager/issues/206)