SP-1249 - Backport of PDI-5252 - Text File Output does not create header when append option is checked (5.0)

Conflicts:
	engine/src/kettle-variables.xml
	ui/src/org/pentaho/di/ui/trans/step/BaseStepDialog.java
	ui/src/org/pentaho/di/ui/trans/steps/textfileoutput/TextFileOutputDialog.java