Issue #786: backend throws a server-error when the xml transformer detects values that contain invalid characters. Frontend updates input before the program is send to the backend.

[PamoCurtis]

This PR prevents that arbitrary code can be generated or executed on the robot by following:

• xml documents which contain invalid values can't be imported into the lab (server-error)
• already imported programs which contain invalid values can't be generated or executed on the robot (server-error)
• blockly validates current input every time the program is processed in some way:
  • open source/sim/debug view
  • refresh source code
  • start simulation
  • execute code on robot
  • save / save as
  • create program link
  • export program
  • download source code