[ci] Add a script to build the frontends.

I expect there to be missing dependencies.
[ci] Pull the install steps out into a script.

Simplify since iirc, python3.4 is now in the official Ubuntu PPAs.
[ci] Add the deadsnakes PPA.

I was wrong...
[ci] Add some qt5 dependencies.
[ci] Try different versions of the qt5 packages.
[ci] Try a PPA for qt5.
[ci] Use the private-dev packages again.
Merge branch 'ci-build'