Add retain basic implementation on GNU/Linux

Retain is saved in file and backup of this file is done every time.
File isn't saved on every PLC cycle to avoid high I/O load.
By default, retain is saved every second and on PLC stop/shutdown.
It's possible to override retain flush period by using CFLAGS option
in project's settings. For example, for 0.5 seconds period
use "-DFILE_RETAIN_SAVE_PERIOD_S=0.5".

#!/bin/bash

INKSCAPE=inkscape

for i in `cat icons.svg |grep -o -e '%%[^%]*%%'|sed 's/%//g'` 
do
 if [ $i.png -nt icons.svg ]; then
 	echo "Skip $i"
 else
	rm  -f $i.png
	echo "$INKSCAPE" icons.svg -z -e $i.png -i $i
	"$INKSCAPE" icons.svg -z -e $i.png -i $i
 fi
done

cp ico24.png brz.png
convert ico*.png brz.ico
rm -f ico*.png