Add Appstream metadata and .libretro file for GNOME Games integration

[Jeremy Bícha]

Impact
======
Virtual Boy games can't be played in the GNOME Games app without a .libretro metadata file.

By adding Appstream addon metadata, we can also make it easy for users to install this addon. All they have to do is open the GNOME Games page in the GNOME Software app and check the box for the addon they want.

The GNOME Games app isn't included in Ubuntu 16.04 LTS so this SRU is only for 18.04 LTS and 18.10.

Test Case 1
===========
1. Enable -proposed updates.
2. Wait a day or so after the package has built so that Appstream metadata is updated.
3. Run these commands:
sudo apt update
pkill gnome-software
gnome-software --details-pkg=gnome-games-app

The GNOME Software app will take a few moments to start this way.
Scroll down to the Add-ons section. There should be a "Beetle VB" add-on. Check the box to install it.

Test Case 2
===========
Verify that all 3 files are installed:
/usr/lib/x86_64-linux-gnu/libretro/mednafen_vb.libretro
/usr/lib/x86_64-linux-gnu/libretro/mednafen_vb_libretro.so
/usr/share/metainfo/libretro-beetle-vb.metainfo.xml

Optional Test Case 3
====================
If you have a Virtual Boy rom, you can put it in the root of your home directory and see if GNOME Games can play the file. I don't have any Virtual Boy roms.

Note that GNOME Games requires tracker to be installed. You must log out and log back in after installing tracker if it was not already installed.

Regression Potential
====================
This adds 2 metadata files. We also convert the install file to use dh-exec which is why the second test case verifies the install of *3* files instead of just 2.

dh-exec
=======
The debian/*install files use an unusual shell script style instead of the much more common dh-exec.

I briefly talked to the Debian maintainer about this issue in 2017. He said he moved from dh-exec to sh because a Debian maintainer advised him to.

I am switching from sh to dh-exec because it's what I did in Ubuntu 19.04 and because my opinion is that the dh-exec style is easier to read and understand so we are less likely to make a mistake.

https://codesearch.debian.net/search?q=path%3Adebian%2F.*%5C.install+MULTIARCH&perpkg=1
https://codesearch.debian.net/search?q=path%3Adebian%2F.*%5C.install+%23%5C%21%2Fbin%2Fsh&perpkg=1