Modified FilePluginProvider to use FSNodes (instead of raw filenames / paths) in its API

Files:
modified: /scummvm/trunk/backends/plugins/posix/posix-provider.cpp (try)
modified: /scummvm/trunk/backends/plugins/sdl/sdl-provider.cpp (try)
modified: /scummvm/trunk/backends/plugins/win32/win32-provider.cpp (try)
modified: /scummvm/trunk/base/plugins.cpp (try)
modified: /scummvm/trunk/backends/plugins/posix/posix-provider.h (try)
modified: /scummvm/trunk/backends/plugins/sdl/sdl-provider.h (try)
modified: /scummvm/trunk/backends/plugins/win32/win32-provider.h (try)
modified: /scummvm/trunk/backends/plugins/dc/dc-provider.cpp (try)
modified: /scummvm/trunk/base/plugins.h (try)
modified: /scummvm/trunk/backends/plugins/dc/dc-provider.h (try)