Shortcut for getting current signal

[thomasaarholt]

At the moment, s = ui.get_selected_signal(). Could I propose changing this to s = ui.get_current_signal(), copying matplotlib behaviour and allowing for the acronym-method s = ui.gcs(). In matplotlib, one normally gets ahold of the current figure by plt.gcf(), so something similar would be welcome. The current call is quite long, which isn't ideal.