Clients::Studio: added LinkItem class, which represents connection

    * src/clients/studio/Makefile.am: added implementation file to a build
      file list.
    * src/clients/studio/elementitem.cpp: minor, corrected indentation.
    * src/clients/studio/elementitem.h:
    * src/clients/studio/point.h: moved Point class to a separated file,
      because this class is (and will be) useful also in other places.
    * src/clients/studio/linkitem.{cpp|.h}: link class implementation.
      This class represents connections between pads in element's blocks.
    * src/clients/studio/paditem.{cpp|h}: added mouse events handlers
      (but there is a lot of to do in this area, it's just a draft;)