Rename plugin modules with "Plugin_" preface

Better organization, IMO
Add full ExifTool plugin support

DISCLAIMER: by "add full ExifTool plugin support", I mean that ExifTool
is now provided as an official plugin.  If missing at run-time, it can
be downloaded from my server.  Its version number is checked (by reading
stdout and passing the "-ver" flag), and it can be enabled/disabled from
the plugin manager.  There is also a new global flag called
g_ExifToolEnabled that individual functions can use to test availability
before requesting ExifTool actions.

All that said, the plugin doesn't actually DO anything yet.  That will
come later.