various improvements in the build process for Windows

* verify that Perl and NSIS are installed before the MSVC build
* install in "Program Files (x86)" for a 32-bit build on 64-bit OS
* refuse to install the 64-bit binaries on a 32-bit OS
link with the multi-threaded dynamic CRT for Windows/MSVC builds
install the MSVC runtime if it is not present on the target machine

The appropriate runtime is downloaded at the time of building the
installer and bundled with it i.e. no separate download is required.
update README with details of Qt5 port
switch to using qt5 submodules
include gnuwin32 for the Windows build