Bug 408027 - [sidebar] Add ARIA semantics to sidebar navigator
-First steps. Doesn't behave correctly in NVDA yet.
-Navigators have 'tree role
-Rows have 'treeitem' role
-File/Folder nodes have 'link' role
Bug 408027 - explorerNavHandler: add role='presentation' and set the aria-activedescendant
grab stuck because mouse up is ignored on embedded widgets