sigrok-cross-mingw: Drop MSI Python installer, ship only libs/modules.

sigrok-cross-mingw: Drop MSI Python installer, ship only libs/modules.

Until now we used to include the official Python MSI Windows installer
in the installers for our frontends (sigrok-cli and PulseView), and it
was run during the installation of the sigrok frontend. Python was also
hardcoded to be installed into c:\python32.

Change this to only ship python32.dll and python32.zip now, which are
simply copied into the install location of the frontend (e.g.
"c:\Program Files (x86)\sigrok\PulseView". The python32.zip file
contains a few DLLs and libs, and all Python stdlib modules.

This has various advantages:

 - There are no longer issues if a user has other Python versions
   installed on the same OS (e.g. a different Python 3 version and/or
   Python 2.x versions). The sigrok frontends will always use _only_ the
   local Python from their install dir and ignore any others.

 - We (or rather the Python MSI installer) no longer create any registry
   entries, potentially change the PATH env. variable, change file
   associations, add Python to the Windows "list of installed software",
   create a Start Menu entry for Python, or do any other things that affect
   the system as a whole. After an uninstall of the respective sigrok
   frontend no traces of (our local) Python will be left on the system.

 - We no longer need to hardcode any path for the Python install. The
   required files will always be installed whereever the sigrok frontend
   is installed, no matter wheter it's on c:\ or any other drive.

 - The installation time is reduced (we no longer need to run the
   official MSI installer) and the size of our NSIS installers is also
   reduced quite a bit (python32.dll + python32.zip are a lot smaller
   than the full Python MSI installer).

 - Frontend load times might even be reduced a bit, since Python will now
   only scan for modules in the two locations we provide (the
   python32.zip file and the decoders/ subdirectory).

Add srd_decodersdir.patch which replaces the (now obsolete) patches
pulseview_decodersdir.patch and sigrok_cli_decodersdir.patch.