Beanshell to enable save button only upon modification of script.
Save button disables after script is successfully saved. Also, save
button will be disabled when there are no undoable changes.
ScriptEditorForBeanShell registers listener for unsaved changes.
PlainSourceView triggers listener calls upon modification of script or
after a successful save, which lets to enable/ disable the save button
accordingly.

Change-Id: I32a2fc473924a7c85cdd6004637ab6a0b60acf38
Reviewed-on: https://gerrit.libreoffice.org/69046
Tested-by: Jenkins
Reviewed-by: Samuel Mehrbrodt <Samuel.Mehrbrodt@cib.de>