hotkey for toggling tilda transparency 

Changing the terminal from transparent to non-transparent currently takes 4 mouse clicks. Between well-lit environments where transparency makes it difficult to see and wanting to read or compare terminal content against background information, I found it to be something one does quite often. 
My knowledge of c is very limited (not to mention GTK) so I apologize if something is wrong.  Properly `toggle_transparency` should not have that much redundant code with the wizard.c `check_enable_transparency_toggled_cb`, but this seemed easier.