Issue #381 clickable autocomplete

Made autocomplete to work with mouse.
I know the issue was created just a few hours ago. And was not yet approved by p-e-w. But I decided to implement it anyway. 
This is the weirdest construct I have ever seen in Vala. I am completely unfamiliar with such syntax. How does this compile?!? Can you enlighten me?

It's not a method (no parentheses, unless the ones around `selected_text` are somehow counted), it's not a property (no get/set blocks), so what is it?