[UIFe] Update the Ubiquity slideshow (Xubuntu)

Please update the Ubiquity slideshow for final changes to the Xubuntu slideshow.

I've created a branch that has all the proposed changes and done a merge request:
https://code.launchpad.net/~knome/ubiquity-slideshow-ubuntu/uife-bug-1294619/+merge/211707

The changes are:
* Switch two screenshots to final versions
* Add two new images to use in the slideshow (one major, one minor change)
* Minor visual updates (change color of panel highlight, make an element look like a speech bubble)
* CSS cleanup

Changes in the branch that are not subject to UIFe:
* Minor text changes
* Update translation templates (changes only in the Xubuntu template)

The Xubuntu documentation is built in a way that nothing needs to be udpated if the installer slideshow changes, but if anything needs to be updated, the team is happy to do any work required.

We will also need sponsoring for this change, so if possible, please upload the new package version if/as you approve this UIFe.