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To now, each property hat an _invalidationLevel field that was evaluated by processors when the property had changed in order to determine what has to be done. However, since properties could also be owned by other classes, this design was semantically misleading.
Therefore, it was removed with this commit and replaced by the invalidation map of each processor. Now, this per-processor mapping of property -> invalidation level is managed by the processor itself. Furthermore, the invalidation level is no longer setup during property creation but during AbstractProcessor::addProperty(), which also makes much more sense.

ATTENTION: Due to these intrusive API changes, the code of all processors and other classes handling properties needs to be changed. As a reminder, the implementation of addProperty() also now takes a reference instead of a pointer, so that old code does no longer compile.

refs #542

C++ implicit conversion rules made ShaderManager::loadSeparate() ambiguous in some cases. So I decided to cut loose ends and completely refactored loading with standard version into ShaderManager::load() and loading with custom version into ShaderManager::loadWithCustomGlslVersion().
ShaderManager::loadSeparate() is no longer available!

Refactoring AbstractProcessor::process() for clearer semantics and better and more uniform handling of invalidation levels:
 * AbstractProcessor::process() now calls updateShader(), updateProperties() and/or updateResult() with respect to the current invalidation level
 * each processor shall no longer override process() but the updateXYZ() methods, at minimum updateResult()
 * AbstractProcessor::process() takes care of (un)locking the processor itself (no need to do this from the outside anymore)

Further implicit changes:
 * Removed redundant HasPropertyCollection::updateProperties()

* Moving to Apache 2.0 license
* Updated AUTHORS.txt

* Split up ProcessorDecoratorShading in ProcessorDecoratorShading inheriting from ProcessorDecoratorGradient
* Introducing GlGradientVolumeGenerator processor in preprocessing module