Add ability to get text from ControlTextBox via python and infolabels

Added GetDescription() to CGUITextBox which allows xbmc.getInfoLabel("Control.GetLabel(100)") to return TextBox texts.
Add getText() to ControlTextBox which allows <control>.getText() to return the textbox text in python.

Everything works fine, but I have to wonder if there is some reason this was never done before because it was so simple :)