The idea behind NodesWatcher was that it was the central place
to catch node related signals, so that users didn't need to
connect to each individual node and the nodes didn't need to
be QObjects. Somehow Nodes ended up being QObjects anyway.

Both the recently added ProjectTree and the FlatModels consume
the signals the NodesWatcher sends. Unfortunately there's a
ordering dependency between the ProjectTree and the FlatModels.

This patch removes all NodesWatcher and instead makes the
ProjectTree singleton the emitter of various project tree
related signals. The ProjectTree also ensures that the ordering
between the FlatModel and itself is taken into account.

And it makes Node not derive from QObject, saving some memory
in that process.

Task-number: QTCREATORBUG-13756
Change-Id: I8b0d357863f1dc1d2d440ce8172502594138b9fb
Reviewed-by: Tobias Hunger <tobias.hunger@theqtcompany.com>