Add option for profiles

Before, the Webslice plasmoids were all like private windows in a browser : no data was stored, and each time your machine would reboot, you had to log in again.
So, I added an experimental option to allow the storage of data, and even to be able to change the profile folder name, to allow different logins to the same site at the same time (across multiple Webslices, of course).
This option is experimental because of a bug in Qt. So please read what is written around that option !

Also, updated German translation, thanks to Roland, and a new Dutch translation, thanks to Heimen Stoffels !!!