Adapt TargetGenerator to be usable from Maven source folder

[MartinLeidig]

The TargetGenerator? has to be adapted allowing to generate all targets from the source folder during a production mode build. Therefor it has to be able to handle that resources, like images can't be accessed in extracted form, the cache dir is located outside of the docroot, etc.

[MartinLeidig]

Additionally added opportunity to set build mode (as buildtime properties are only available in target).