Cabal build support

[geekosaur]

Description

Add support for cabal builds based on presence of a cabal.project file, analogous to our support for stack builds based on stack.yaml.

Checklist

• I've read CONTRIBUTING.md

• I've confirmed these changes don't belong in xmonad-contrib instead

• I've considered how to best test these changes (property, unit,
  manually, ...) and concluded: needs manual testing

• I updated the CHANGES.md file

[geekosaur]

I need to do a bit more testing, since this overrides the INSTALL.md workflow while adding a new one.

[geekosaur]

I talked about this a bit on IRC, but I'm expanding on it and recording it here.

I'm going to abandon the cabal.project-driven implementation, since the existing cabal install --lib stuff already uses it (and must) in the case where xmonad and xmonad-contrib are to be installed from git. Instead, I want to use an xmonad.cabal file, which enables the use of additional dependencies. It does mean the file must be more complete than a stack.yaml, but that is unavoidable: stack and cabal have different divisions of labor between their config files vs. their cabal files. But it solves the problem that it is impossible to have additional dependencies (dbus, utf8-string, etc.) without vendoring them in cabal.project with the original implementation.

One open question is whether to stick to a specific cabal file as described above, or do what cabal does and search for any *.cabal file in the build directory. My original proposal goes for the simple solution, but @liskin said he found this odd.