Core: added a few methods which helps to get info about gstreamer plugins
and factories

        * src/core/plugininfo.{h|cpp}: added methods returning plugin's
          info and metadata of Gst::FactoryElement
Clients::Studio: added windows which displaying info about plugins and
element factories

        * clients/studio/Makefile.am: updated build list
        * clients/studio/buildablegtk.h: base abstract class for all
          objects containing gtkbuilder object. It provides a method for
          easy and safe getting gtk objects.
        * clients/studio/factoryinfo.{cpp|h}: class displaying info about
          specyfic element factory (for now only metadata is displayed).
        * clients/studio/gstreamer-studio.cpp: set default log level to
          INFO - it's helpful during application development
        * clients/studio/mainwindow.h: mainwindow class is now derived
          from BuildableGtk class
        * clients/studio/plugininfowindow.{cpp|h}: class responsible for
          showing info about gstreamer plugins
        * clients/studio/twocolumnmodel.h: model class for GtkTreeView
          with two columns
        * clielts/studio/ui/plugininfo.glade: gui for info windows