properly cleanup registry entries when uninstalling on 64-bit Windows
fixed #1863 via wkhtmltopdf/qt@ccffd0d9842e06ed1521fa669faec2c50e6bae71
block access to local files on Windows when an invalid URL is used

If you use an absolute path e.g. C:\TEST.PNG it is incorrectly
interpreted as an URL with the scheme as "C". This shows up as a
warning which was added in QTBUG-17731:

QNetworkAccessFileBackendFactory: URL has no schema set, use file:// for files

However, we should simply not process such URLs similar to what was
done for Qt5 via qtproject/qtbase@9581c90395086932d2e853864bf962b0896597d5.

Fixes #1639
bump version to 0.13.0-alpha
update README with details of Qt5 port
remove static linkage to text codecs not present/required for Qt5
allow custom variables to be used when building dependent libraries
remove qt submodule and use qt5 submodules for qtbase and qtwebkit
include gnuwin32 for the Windows build
download and compile ICU for the Windows MSVC build

As Cygwin is required for building ICU, it is now a dependency for
the Windows MSVC build.
get qtbase to compile
get qtsvg and qtxmlpatterns to compile
try to get qtwebkit working on Windows