Fix longstanding bug with 'platform_offset'

[drewdoggy]

Re: https://community.ultimaker.com/topic/34232-how-do-i-add-my-printers-logo-to-the-cura-bed/?do=findComment&comment=293900

The x, y, z offset is read in from the user .cfg file as a string, but the code expects a list of floats. The check for 3 items always fails with sensible data as it returns the number of characters in the string.

I am checking if the instance of offset is a string because I believe this offset is also provided by .json printer definition files as the correct data type and we don't want to break that.