Clients::Studio: added info panel with selected element

    * src/clients/studio/Makefile.am: added panel sources to a build.
    * src/clients/studio/elementinfo.{cpp|h}: panel's sources, simple
      method for updating element's description.
    * src/clients/studio/mainwindow.{cpp|h}: added panel to mainwindow's
      source.
    * src/clients/studio/ui/mainwindow.glade: glade file updated.
Clients::Studio: improved main ui

    * src/clients/studio/ui/mainwindow.glade: main gui is based on a paned
      containers.