* Fix minor bugs and improve code formatting of Python build tool
* Prepend $PATH to give it higher priority
* Invoke WiX tools through variable to ensure correct path
* Make build script ensure pacman dependencies are installed
* pacman -Sy causes dependency break on AppVeyor
* Skip deps version check and don't show progress bar
Port the current installer infrastructure from the gtk-sharp-2-12
branch, adapting it as needed. Credits go to Duncan Mak for
maintaining the 2.12 installer in the past few years.
When building on Windows, if the WiX toolset is available, the msi is
generated automatically at the end of the build.