Allow shop owner to control shop specific i18n

currently all sf localisation comes from shop_xx.properties.xml file which may be troublesome when shop owner wishes to modify few labels.

TD:
1) improve i18n via Shop custom attribute shop_xx.properties.xml.
2) this should NOT be complete copy of shop_xx.properties.xml just overwritten values
3) values should be java style properties NVP (should be loadable via Properties.load())
e.g.
label1=Label1
label2=Label2