speedo: Improve building of the w32 installer.

* build-aux/speedo.mk: Change name of build directory to PLAY.
Improve the dist-source target.
* build-aux/speedo/w32/gdk-pixbuf-loaders.cache: Add a blank
line (plus comment).
* build-aux/speedo/w32/inst.nsi: Change name of file to gnupg-w32-*.
Install more tools.

gdk-pixbuf-loaders.cache needs to end with an extra LF or the
gdk-pixbuf is not able to read the last entry. The final comment is
to make our git sanity checks happy.

Running

make -f build-aux/speedo.mk \

TARGETOS=w32 TARBALLS=~/tarballs installer

does now create a working installer. After removing dirmngr from the
installation GPA kind of works. There are remaining problems with
dirmngr and scdaemon which will be fixed soon.

Running

make -f build-aux/speedo.mk \

TARGETOS=w32 TARBALLS=~/tarballs dist-source

creates an xz compressed tarball with all the sources used to build
the installer. Distributing this tarball along with the installer is
sufficient to comply with the GPL. Well, some more instructions
should be given in the readme files.