build: Add support for building AppImages under linux

appimage/apprun.sh

@@ -0,0 +1,4 @@
+#!/bin/sh
+if [ "${APPIMAGE}" != "" ]; then
+	XDG_DATA_DIRS="${APPDIR}/usr/share:${XDG_DATA_DIRS}" "${APPDIR}/usr/bin/Vita3K" $@
+fi

appimage/build.sh

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+#!/bin/sh
+
+echo "Executing linuxdeploy with cmdline: LDAI_VERBOSE=1 $@"
+
+LDAI_VERBOSE=1 $@

appimage/build_updater.sh

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+#!/bin/sh
+
+# Note: The documentation at https://github.com/linuxdeploy/linuxdeploy-plugin-appimage/blob/master/README.md for embedding update info is wrong.
+# The correct env-variable name to use is UPDATE_INFORMATION, and *not* LDAI_UPDATE_INFORMATION.
+
+echo "Executing linuxdeploy with cmdline: LDAI_VERBOSE=1 UPDATE_INFORMATION=\"gh-releases-zsync|Vita3K|Vita3K|latest|Vita3K-x86_64.AppImage.zsync\" $@"
+
+LDAI_VERBOSE=1 UPDATE_INFORMATION="gh-releases-zsync|Vita3K|Vita3K|latest|Vita3K-x86_64.AppImage.zsync" $@

appimage/vita3k.desktop

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+[Desktop Entry]
+Type=Application
+Name=Vita3K
+Icon=vita3k
+Categories=Game;Emulator;X-None;