Change it into a singleton that listens for device changes
and emits signals when devices are added or removed.

Merge-request: 1949
Reviewed-by: Thorbjørn Lindeijer <thorbjorn.lindeijer@nokia.com>

Reviewed-by: Oswald Buddenhagen

TODO: only transfer deltas

this makes the i/o part of the evaluator thread-safe.
for safety, assert absolute paths in IoUtils::exists().

on the way, i "stole" some code i wrote for KDE. there have been no
copyright-worthy contributions from others to it, so this is legal.

somewhat faster again

now items have no vtable any more

it's overengineered for our purpose and would just get in the way of
planned optimizations.

use a hash of magic variable names to ints, which allows fast lookup
followed by a jump table dispatch

Reviewed-by: Thorbjørn Lindeijer

Move dependencies up.

To be able to share TrkDevices between Debugger and Qt4ProjectManager.

The compiler used in Pulse does not support return-type and
init-self compiler flags.

Reviewed-by: kh1

Reviewed-by: hjk

previously.