integrate pk3 support into the context menu and file associations

The in-FM menu was pretty straightforward.

For the Windows Explorer context menu integration, the 7-zip.dll needs to be
built as 64-bit (to match my OS flavor). And a few other things that I changed:

1) Instead of this command line for build:
   nmake NEW_COMPILER=1 MY_STATIC_LINK=1

   I used this:
   nmake NEW_COMPILER=1 MY_STATIC_LINK=1 CPU=AMD64 NO_BUFFEROVERFLOWU=1

   Although as far as I can tell this doesn't change anything except when
   compiling asm files.

2) At one point I was trying things with a "real" version of 7-zip already
   installed, and manually editing some registry keys/values to point the
   context menu handler at my new DLL. Then I later uninstalled the "real"
   7-zip and rebooted before proceeding. It's not clear if the uninstall
   and/or reboot was necessary.

3) When choosing menu options that affect Windows Explorer shell integration
   (context menu or file associations), I _think_ I had to be running FM as
   administrator. I was juggling a couple of other tests/changes at the time
   though so not sure. Need to re-check.

Super scientific!