Display 'already running' msg using yad (#157)

Display the "gpartedbin is already running" message as a graphical error box
using yad (Yet Another Dialog) if it's available, in addition to printing to
stdout.

GParted is a graphical tool so most users will not be launching the tool from a
terminal, so displaying a graphical dialog error box gives the user more
clarity.

Note: This commit does not add 'yad' as a dependency of GParted.

Also, the existing stdout message is not yet translated.

Closes #157 - Display 'GParted already running' error graphically

gparted.in

@@ -27,12 +27,22 @@
 #  along with GParted.  If not, see <http://www.gnu.org/licenses/>.
 #
 
+display_msg() {
+	MESSAGE="$1"
+	TITLE="$2"
+	printf "$MESSAGE\n"
+	which yad >/dev/null
+	yad_exists=$?
+	if [ $yad_exists -eq 0 ]; then
+		yad --center --width 300 --title="$TITLE" --button="OK:0" --text "$MESSAGE"
+	fi
+}
+
 #
 #  Only permit one instance of GParted to execute at a time
 #
 if pidof gpartedbin 1> /dev/null; then
-	echo "The process gpartedbin is already running."
-	echo "Only one gpartedbin process is permitted."
+	display_msg "The process gpartedbin is already running.\n\nOnly one gpartedbin process is permitted."
 	exit 1
 fi
 
@@ -51,7 +61,7 @@ if test "x`id -u`" != "x0"; then
 	#  privileges.
 	#
 	if test "x@gksuprog@" = "x"; then
-		echo "Root privileges are required for running gparted."
+		display_msg "Root privileges are required for running gparted."
 		$BASE_CMD
 		exit 1
 	fi