Improve URL backward-compatibility after introduction of Nested Spaces

Ok I brainstormed with tmortagne (and softec complained before but I didn't follow him at the time and I was wrong) and we agreed that it's better for backward-compatibility to do this:

• If the first segment is not an action name, then consider that the action is "view" (whether showview is true or false)
• If the first segment is an action name then always consider that it represents an action (whether showview is true or false). e.g. if the first space is called "view" then "view/view" will need to be used to produce a view URL for it.
• Modify the URL serialization to output "view/<first space name>" when the first space name is an action name and the action is View (and showview = false)

Right now, we have the limitations mentioned in the Release Notes of 7.2: http://www.xwiki.org/xwiki/bin/view/ReleaseNotes/ReleaseNotesXWiki72M1#HURLs (caused by XWIKI-12169) and this is causing XWIKI-12834 for example.