Add more variables for the splash.svg image (Closes: 1015782)

The request asked for 5 additional variables to be resolved for the SVG
images:
Added: LB_ISO_APPLICATION, LB_ISO_PUBLISHER and LB_LINUX_PACKAGES
Not added: LB_ISO_PREPARER: xorriso recommends not changing this
variable
Not added: LB_ISO_VOLUME: is can contain '@ISOVOLUME_TS@' which resolves
to a timestamp and would need additional handling