added preliminary support for tags to application variables which can be searched for and used to generate virtual modules

fixed replacing Array and ScalarAccessors, which was broken after moving to the NDRegisterAccessorBridge-based accessors

fixed poll-type inputs using no longer non-blocking read after the switch to the NDRegisterAccessorBridge-based accessors

Rebased the ScalarAccessor and ArrayAccessor classes onto the mtca4u::NDRegisterAccessorBridge class.
Known problems present: code compiles but tests do not run

Removed the macros for defining accessors and introduced convenience classes instead. This adds slightly more syntactic sugar, but also allows more flexibility and seems to be a more coherent interface.

This commit incompatibly breaks the interface with the previous versions. Since ApplicationCore is not yet used in any productive environment, no compatibility is provided.

- Default constructor for Accessor exists now
- A function Accessor::replace() allows to assign a new accessor
- For clarity/safety: copying accessors using the assignment operator is now forbidden
- The destructor of the accessor unregisters it from the module (to allow temporary accessors)

Added description field to the variables. It is currently ignored and will be added to the XML file in a next step.