Make the Gantt zoom images more accessible

* Correct the alt text
* Add text links next to the images
* Size the images in em to support browser scaling

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@3757 e93f8b46-1217-0410-a6f0-8f06a7374b81

config/locales/en.yml

@@ -871,6 +871,8 @@ en:
   text_wiki_page_destroy_children: "Delete child pages and all their descendants"
   text_wiki_page_reassign_children: "Reassign child pages to this parent page"
   text_own_membership_delete_confirmation: "You are about to remove some or all of your permissions and may no longer be able to edit this project after that.\nAre you sure you want to continue?"
+  text_zoom_in: Zoom in
+  text_zoom_out: Zoom out
   
   default_role_manager: Manager
   default_role_developper: Developer