MDL-53048 form: Rewrite passwordunmask

This version:
* should work with the Behat Goutte driver
* should not suffer from password autofill anxiety
* should allow unmasking (and masking) of a password
* should allow editing of passwords in either masked, or unmasked form

AMOS BEGIN
 MOV [revealpassword,core_form],[passwordunmaskrevealhint,core_form]
AMOS END

lib/behat/classes/partial_named_selector.php

@@ -186,6 +186,11 @@ XPATH
             'filemanager' => <<<XPATH
 .//*[@data-fieldtype = 'filemanager' or @data-fieldtype = 'filepicker']
     /descendant::input[@id = //label[contains(normalize-space(string(.)), %locator%)]/@for]
+XPATH
+        ,
+             'passwordunmask' => <<<XPATH
+.//*[@data-passwordunmask='wrapper']
+    /descendant::input[@id = //label[contains(normalize-space(string(.)), %locator%)]/@for]
 XPATH
         ],
     ];