Webform Settings local task should not appear unless the fieldmap is a webform fieldmap

Current behavior:
sf_webform_menu defines a local task "Webform Settings" that appears for all fieldmaps.

Expected behavior:
local task "Webform Settings" should only appear where relevant, ie. when the fieldmap is a node_webform fieldmap.

Attached patch implements an access callback for this menu item to check whether the fieldmap given by the page argument is a webform fieldmap, and only allows access when relevant.