TRANSLATE-2244: Embed translate5 guide video in help window
Embed the translate5 guide videos as iframe in the help window. The videos are either in german or english, they are chosen automatically depending on the GUI interface. A list of links to jump to specific parrs of the videos are provided.
TRANSLATE-1237: Exported xliff 2.1 is not valid
The XLF 2.1 output is now valid (validated against https://okapi-lynx.appspot.com/validation).
TRANSLATE-2243: Task properties panel stays enabled without selected task
Sometimes the task properties panel was enabled even when there is no task selected in the project tasks grid.