gstreamer recipes: disable plugins that can't be built on UWP

Things left after this are:

• Update FFmpeg to the latest upstream version (4.3 is being prepped), and then use our meson port in the ffmpeg recipe
• Disable recipe plugins in packages/* so we can output a tarball package

commit 55de9dc0:

libnice.recipe: Port to WinRT/UWP

commit 906b44c8:

custom.py: Make enable_plugin work like disable_plugin
If the `option` argument is not specified, `disable_plugin()` sets it
to `plugin`, but `enable_plugin()` sets it to `variant`.

This made sense in the very beginning because of some edge cases such
as v4l2 and pulse, but there's just as many cases where the option is
the same as the plugin name and that is true for the majority of
plugins, so make it consistent.

commit fb1fa5a4:

gst-plugins-base-1.0.recipe: Disable features on UWP
Currently none of the features that require external dependencies can
be built on UWP. This will change with time, of course.

commit cca01c4d:

gst-plugins-good-1.0.recipe: Disable features on UWP
Currently only libvpx can be built on UWP, so disable the rest.

commit 3a465e68:

gst-plugins-bad-1.0.recipe: Disable features on UWP
Only the following external deps currently build on UWP:

libnice, libsrtp, libusrsctp, openh264, openssl

commit e0010aa7:

gst-plugins-ugly-1.0.recipe: Disable features on UWP
x264 can be enabled, but it needs our meson port to be updated.