ProjectExplorer: Remove hack for virtual folders

Previously virtual folders, that is the "Sources", "Headers" folders used a
hack. This patch removes that hack, by introducing the following changes
- The FlatModel and the ProjectExplorer::Nodes now don't require path() to
  be unique. Thus allowing the virtual folders to all return the same for
  path(). [1]

- Introducing a new node type "VirtualFolder" which is sorted according to
  a priority.

- Introducing a few new virtuals for displayName and toolip(), which can
  be overriden.

[1] Note that all the project managers do require path() to be unique for
some types of nodes.

That also fixes:
Task-number: QTCREATORBUG-7100

Change-Id: I76b730f4c4254e2894467603bbe9a30e356a0bcc
Reviewed-by: Tobias Hunger <tobias.hunger@nokia.com>
Reviewed-by: Daniel Teske <daniel.teske@nokia.com>