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

Use mtca4u::setDMapFilePath() instead of the BackendFactory's member function. Since all applications will need it, the corresponding header file is now included in ApplicationCore.h

Introduced the DeviceModule class which provides an interface to obtain VariableNetworkNodes for device registers

Introduced the VariableNetwork describing the connections of variables/accessors before things get actually instantiated. This is needed to generate an XML file containing the published variables based on that information.