-> changed some name of files in order to faster recognize them
-> changed a lot of wrong/useless UNSIGNED="false" in some plugin/db/install.xml (I do not care about instances of survey already installed so, no corresponding code is in upgrade.php)
-> added downloadformat for multiselect and rate plugins
-> added $CFG->survey_forcemodifications to each occurrence of $hassubmissions. $CFG->survey_forcemodifications is the new general setting allowing to ignore the deny to change surveys already submitted
-> uniformed the message for the user when he/she try to change a survey already submitted
-> changed some variable name in order to better allow to understan what it represent
-> fixed a but for numeric plugin. When user submitted 0 the submission in PDF reported "Answer refused"
-> fixed a but for numeric plugin. upper and lowed bound were no longer editable.