Unify method to query available terminals

Because of some exceptions such as gnome-terminal and kgx, the ordering
was getting convoluted when considering precedence. Instead of adding
new if statements every time a terminal requires precedence, run only
add a new entry to the for loop. Also allows the user to enforce a
terminal with environment variable TERMINAL.

app-menu/qubes-run-terminal

@@ -1,32 +1,36 @@
 #!/bin/sh
 # Try to find a terminal emulator that's installed and run it.
+set -eu
 
-is_command() {
-    # bogus warning from ShellCheck < 0.5.0
-    # shellcheck disable=SC2039
-    type "$1" >/dev/null 2>&1
+is_command(){
+    command -v "$1" >/dev/null
 }
 
-if is_command x-terminal-emulator; then
-    exec x-terminal-emulator
-fi
-
-if is_command ptyxis; then
-    exec ptyxis
-fi
-
-if is_command gnome-terminal; then
-    exec qubes-run-gnome-terminal
-fi
+reassign_term(){
+    case "$1" in
+        gnome-terminal) terminal="qubes-run-$1";;
+        kgx) terminal="qubes-run-gnome-console";;
+    esac
+}
 
-if is_command kgx; then
-    exec qubes-run-gnome-console
+if is_command "${TERMINAL:-}"; then
+    terminal="${TERMINAL}"
+    reassign_term "$terminal"
+    if is_command "$terminal"; then
+        exec "${terminal}"
+    fi
 fi
 
-for terminal in xfce4-terminal konsole urxvt rxvt termit terminator Eterm aterm roxterm termite lxterminal mate-terminal terminology st xterm; do
-    if is_command "$terminal" ; then
+for terminal in \
+    x-terminal-emulator ptyxis gnome-terminal kgx xfce4-terminal konsole \
+    urxvt rxvt termit terminator Eterm aterm roxterm termite lxterminal \
+    mate-terminal terminology st lxterm xterm
+do
+    reassign_term "$terminal"
+    if is_command "$terminal"; then
         exec "$terminal"
     fi
 done
 
 echo "ERROR: No suitable terminal found." >&2
+exit 1