Core: added PluginInfo class providing simple way to get plugin's info

        * configure.ac: added gstreamermm-1.0 to required library's list
        * src/core/Makefile.am: added to makefile class files, gstreamermm
          dependencies
        * src/core/plugininfo.{cpp|h}: PluginInfo class implementation
Clients::Studio: added treeview with available GStreamer elements
to a mainwindow

        * src/clients/studio/Makefile.am: added necessary source files,
          dependencies
        * src/clients/studio/gstreamer-studio.cpp: call gstreamermm init
          method, which initializes glibmm and gstreamermm library
        * src/clients/studio/mainwindow.cpp: getting treeview from glade
          file, creating plugins_inspector object
        * src/clients/studio/mainwindow.h: created field stored pointer to
          a plugin inspector
        * src/clients/studio/plugininspector.{cpp|h}: class responsible
          for initializing/(in the future) updating model of treeview
          containig gstreamer elements list
        * src/clients/studio/ui/mainwindow.glade: added treeview with
          gstreamer elements