XWIKI-10662: The scrollbar of the TreeView is not visible under Flamingo and IE-10

- Fixed the IE10 issues by explicitly giving the width for elements that held the background-images
- Regrouped the AllDocs Tree properties in one place in order to be more organised and also to control the order they are applied
- Retested all the properties on Chrome36, Firefox31, Safari7, IE10 and IE11