Add install rules and metadata for GTK/Linux

This allows to install Flips on Linux desktops, with all the modern
bells and whistles, like SVG icons, AppStream metadata, a flatpak
manifest, and the com.github.Alcaro.Flips reverse domain name app ID.

flatpak/com.github.Alcaro.Flips.json

@@ -0,0 +1,34 @@
+{
+    "app-id" : "com.github.Alcaro.Flips",
+    "runtime" : "org.gnome.Platform",
+    "runtime-version" : "3.34",
+    "sdk" : "org.gnome.Sdk",
+    "command" : "flips",
+    "finish-args" : [
+        /* X11 + XShm access */
+        "--share=ipc",
+        "--socket=x11",
+        /* Wayland access */
+        "--socket=wayland",
+        /* OpenGL + DRI access */
+        "--device=dri",
+        /* Needed to find the ROMs and patches */
+        "--filesystem=host:ro"
+    ],
+    "modules" : [
+        {
+            "name" : "flips",
+            "buildsystem" : "simple",
+            "build-commands": [
+                "sh make.sh --flatpak=yes",
+                "sh make.sh --flatpak=yes --install=yes"
+            ],
+            "sources" : [
+                {
+                    "type" : "git",
+                    "url" : "https://github.com/Alcaro/Flips.git"
+                }
+            ]
+        }
+    ]
+}