Add LXQt support

[tsimonq2]

Add support for the LXQt desktop.

Soon (not now) we'll also have the lubuntu-qt-desktop package to use. I'm personally leading that effort and I'll comment here when it's ready.

But for the time being, for Yakkety, we could have LXQt support. The lxqt package works fine.

Speaking of this, I created a PR adding Yakkety support. (#2742)

[dnschneid]

There's a WIP in #2525, but it (along with the Mate target #2532) still have unresolved installation issues.

[drinkcat]

Let's track in #2525.