Add manifest for building as a flatpak

includes the following:

- org.DolphinEmu.dolphin-emu.metainfo.xml.in this file contains general
  information and metadata that is used by most linux based app stores
  (kde discover, gnome software, flathub.org, etc) to showcase dolphin.
- fill_release_node.sh a script to automatically fill in the release
  information in the metainfo file when building the flatpak.
- SDL2.json manifest to build the vendored version of SDL from the
  Exports directory, it's only temporarily needed until the kde runtime
  is updated with the current SDL2 version and should be deleted after.
- org.DolphinEmu.dolphin-emu.yml the manifest itself.

Flatpak/SDL2/SDL2.json

@@ -0,0 +1,22 @@
+{
+  "name": "SDL2",
+  "buildsystem": "autotools",
+  "config-opts": ["--disable-static"],
+  "sources": [
+    {
+      "type": "dir",
+      "path": "../../Externals/SDL/SDL"
+    }
+  ],
+  "cleanup": [ "/bin/sdl2-config",
+               "/include",
+               "/lib/libSDL2.la",
+               "/lib/libSDL2main.a",
+               "/lib/libSDL2main.la",
+               "/lib/libSDL2_test.a",
+               "/lib/libSDL2_test.la",
+               "/lib/cmake",
+               "/share/aclocal",
+               "/lib/pkgconfig"]
+}
+