flatpak: add Intel MediaSDK plugin

flatpak: add Intel MediaSDK plugin

Defines a flatpak manifest that is used to download and build all the
dependencies for MediaSDK 19.1.pre2.  These are built into a flatpak
plugin that can be installed independently from the main HandBrake flatpak.

When the plugin is installed, HandBrake gains the ability to do QSV
encoding with the latest MediaSDK and VA drivers.

When HandBrake is configured with '--flatpak --enable-qsv' and
'make pkg.create.flatpak' is run, both the HandBrake flatpak and the
MediaSDK plugin will be built.

The name of the MediaSDK plugin file is:
fr.handbrake.plugin.IntelMediaSDK-$(HB.version)-$(HB.machine).flatpak

I've also changed the the name of the HandBrake flatpak file to conform
better with flatpak naming:
fr.handbrake.ghb-$(HB.version)-$(HB.machine).flatpak
fr.handbrake.HandBrakeCLI-$(HB.version)-$(HB.machine).flatpak