implement configurable CloudApp API url

With [regenwolken][1] – yes, a shameless self-promote – exists an alternate implementation of the CloudApp API that is completely open source and simple to install.

In addition the user can use HTTPS for secure transport. Actually this is not a feature request of mine (I do not use a Linux Desktop), but of a friend that needs a nice integrated tool like lookit is :)

I hope I made everything right with the GUI part. Unfortunately Unity did not launch the GUI of lookit and I had to use `lookit --preferences`.

[1]: https://github.com/posativ/regenwolken/