Added PropertiesProvider for retrieving project properties.
Added ResourceLocation for finding a resource from an absolute path.
DerivedResourcesIdentifier now uses ResourceLocator.
Merge branch 'master' of git@github.com:cdonnat/padacore.git

Conflicts:
	org.padacore.core/src/org/padacore/core/builder/DerivedResourcesIdentifier.java
Renamed PropertiesProvider to PropertiesManager.
Added methods for managing adaProject in PropertiesManager.
Merge branch 'master' of git@github.com:cdonnat/padacore.git
Updated GprBuildErrObserver to use ResourceLocator. Markers on imported
projects should work by now.
Updated GnatAdaProject test for sources directories.