added survey cover page

Definitely eradicated three variables and deleted corresponding fields from install.xml that were forgotten
Replaced a lot of echo '<img src="'.$OUTPUT->pix_url... with corresponding echo $OUTPUT->pix_icon(
Replaced a lot of echo '<a href="..." title="..."><img src="... with corresponding echo $OUTPUT->action_icon(...
lightweighted the navigation block as each displayed link belongs to the "survey" page
renumbered the keys of the lable of the subtab of the tab "survey"
added a control to deny the display the graph of the frequency report if the required variable is never submitted
added a css style to display $OUTPUT->pix_icon( and $OUTPUT->action_icon( in the same way