Cleanup: FolderManager File object

[jeremypw]

• Inline namespace
• Inline single use functions
• Reorder to static, public, private members, create, construct, public methods, private methods
• Make path construct only
• Set GLib.File only in construct clause
• Load info on construct, including executable attribute
• Start to make naming less ambiguous

[jeremypw]

Changed the header for FolderItem too by mistake, but left it as it is needed anyway.