Plasma theme cache auto-refresh

[Kai Uwe Broulik]

> ok, this is now done. it is in master, please test and let me know if you
> encounter problems.

Cool! Will make things easier in the future, if we overhaul themes again.

Also added a line to the techbase theming page:
"If you do changes to SVG files in your theme, make sure to update the version 
number so Plasma can properly refresh its cache."
:-)