Clients::Studio: added info button for displaying plugin/element info

Double click will be used in the future for adding new element to a model.

    * src/clients/studio/mainwindow.cpp: updated PluginInspector
      constructor call by adding infobutton parameter.
    * src/clients/studio/plugininspector.{cpp|h}: added signal to an info
      button, which opens plugininfo window.
    * src/clients/studio/ui/mainwindow.glade: added info button to ui.