Add install script for AppImage (#1186)

* adjusts to CPU architecture by reading `uname -m`
* `wayland-native` option for Wayland flags
* `remove` option to delete downloaded files
* generates Desktop entry from template
* `--no-sandbox` flag when unprivileged usernamespaces are not available
* downloads various icon sizes to correct locations
  * uses reverse DNS notation so icon can be overridden by icon themes
* add `GenericName`, `Comment`, and `Keywords` to Desktop file
* add `SingleMainWindow` to tell DEs not to suggest opening a new window
* add `StartupWMClass` - probably not required but doesn't hurt either
* update accompanying README instructions
* remove previous example `.desktop` file

install-feishin-appimage

@@ -0,0 +1,74 @@
+#!/bin/sh
+
+dir=$1
+arg=$2
+arch=$(uname -m)
+
+if [ "$arch" != "x86_64" ] && [ "$arch" != "aarch64" ]; then
+	echo "CPU architecture not recognised (not x86_64 or aarch64). Aborting."
+	exit 1
+fi
+
+# workaround if we're not renaming the artifact
+if [ "$arch" = "aarch64" ]; then
+	arch='arm64'
+fi
+
+if [ ! -d "${dir}" ]; then
+	echo "${dir}} is not a directory or does not exist. Please provide an existing directory."
+	exit 1
+fi
+
+localShare="$HOME/.local/share"
+localShareIcons="${localShare}/icons/hicolor"
+
+if [ "${arg}" = "remove" ]; then
+	rm -v \
+		"${localShareIcons}"/512x512/apps/org.jeffvli.feishin.png \
+		"${localShareIcons}"/256x256/apps/org.jeffvli.feishin.png \
+		"${localShareIcons}"/128x128/apps/org.jeffvli.feishin.png \
+		"${localShareIcons}"/64x64/apps/org.jeffvli.feishin.png \
+		"${localShareIcons}"/32x32/apps/org.jeffvli.feishin.png \
+		"${localShare}"/applications/org.jeffvli.feishin.desktop \
+		"${dir}./Feishin-linux-${arch}.AppImage"
+	exit 0
+fi
+
+curl -L --create-dirs --write-out '%{filename_effective}\n' \
+	-o "${dir}/Feishin-linux-${arch}.AppImage" "https://github.com/jeffvli/feishin/releases/latest/download/Feishin-linux-${arch}.AppImage" \
+	-o "${localShareIcons}"/512x512/apps/org.jeffvli.feishin.png 'https://github.com/jeffvli/feishin/blob/development/assets/icons/512x512.png?raw=true' \
+	-o "${localShareIcons}"/256x256/apps/org.jeffvli.feishin.png 'https://github.com/jeffvli/feishin/blob/development/assets/icons/256x256.png?raw=true' \
+	-o "${localShareIcons}"/128x128/apps/org.jeffvli.feishin.png 'https://github.com/jeffvli/feishin/blob/development/assets/icons/128x128.png?raw=true' \
+	-o "${localShareIcons}"/64x64/apps/org.jeffvli.feishin.png 'https://github.com/jeffvli/feishin/blob/development/assets/icons/64x64.png?raw=true' \
+	-o "${localShareIcons}"/32x32/apps/org.jeffvli.feishin.png 'https://github.com/jeffvli/feishin/blob/development/assets/icons/32x32.png?raw=true'
+chmod -v u+x "${dir}/Feishin-linux-${arch}.AppImage"
+
+waylandFlags=''
+if [ "${arg}" = "wayland-native" ]; then
+	waylandFlags=' --enable-features=UseOzonePlatform,WaylandWindowDecorations --ozone-platform-hint=auto'
+fi
+
+# this is for Debian-based kernels and ALT respectively
+# https://unix.stackexchange.com/a/303214/145722
+sandboxFlag=''
+if [ "$(sysctl kernel.unprivileged_userns_clone 2>/dev/null)" = "0" ] \
+	|| [ "$(sysctl kernel.userns_restrict 2>/dev/null)" = "1" ]; then
+	sandboxFlag=' --no-sandbox'
+fi
+
+mkdir -pv "${localShare}"/applications
+cat > "${localShare}"/applications/org.jeffvli.feishin.desktop << EOL
+[Desktop Entry]
+Name=Feishin
+GenericName=Music player
+Exec=${dir}/Feishin-linux-${arch}.AppImage${sandboxFlag}${waylandFlags}
+TryExec=${dir}/Feishin-linux-${arch}.AppImage
+Terminal=false
+Type=Application
+Icon=org.jeffvli.feishin
+StartupWMClass=feishin
+SingleMainWindow=true
+Categories=AudioVideo;Audio;Player;Music;
+Keywords=Navidrome;Jellyfin;Subsonic;OpenSubsonic
+Comment=A player for your self-hosted music server
+EOL