This patch makes a variety of updates to support
building a Natron installer for a Windows Qt5 build.
- Added natron_ prefix to MinGW packages specifically
  built for Natron.
- Fixed MinGW packages so they build with a current
  MinGW installation.
- Updated package dependencies so that each PKGBUILD
  indicates what dependencies it needs to build. This
  allows any of the packages to be built with a
  completely fresh copy of MinGW w/o having to preinstall
  anything other than base-devel. makepkg can install
  everything the package needs from the deps info.
- Added mingw-w64-x86_64-natron-build-deps-qt5 virtual
  package that depends on everything needed to build
  a Natron Qt5 Windows installer. This allows a developer
  to install this one package in a fresh MinGW install
  and have everything needed to build.
- Created a build script that builds all the Natron
  specific packages and stores them in a local pacman
  repo. This makes it easier to build and manage
  Natron packages in a MinGW install.
- Created a GitHub Action script to build all of the
  Natron specific packages, needed for a Qt5 build, on
  a GitHub Windows runner and output the resulting
  pacman repository directory as a zip file. The action
  also caches the repository so it can be used by other
  actions in the future. This is intended as a starting
  point for making it easier to use GitHub Actions for
  Windows builds.