Unify handling of global services, operations, attributes and properties

[meyerj]

GlobalsRepository::Instance() could simply return a pointer to GlobalService::Instance(). I do not see a good reason to use different class instances for attributes and constants. GlobalService could serve as a unified container for:

• global services
• global attributes/constants (formerly in GlobalsRepository)
• global properties (deployment:: implement GlobalsRepository Properties parsing ocl#63)
• global operations/methods

All of these primitives can then be accessed from RTT scripting and the OCL TaskBrowser either explicitly (by global.<name>, with a new keyword global) or implicitly, if the name is not defined in the current context. This would simplify the implementation of orocos-toolchain/ocl#63 and the TaskBrowser in general.

Ultimately, the GlobalService could be replaced by a global TaskContext which even can have peers (global components) and is served by the GlobalEngine.