Reload of control files on file changes

[romangg]

We currently only read the control files when the config hash changes (i.e. when the composition of outputs changes) but in some corner cases the control files might need to be reloaded even without such a change:

1. When Disman is run in-process and some other process changes the config values.
2. When the files were changed manually by the user.
3. When in a parallel D-Bus/desktop session the control files were changed with a separate Disman backend service instance.

Cases 1 and 2 could be just declared as non-supported but 3 is a real issue albeit a very rare one.

We could implement a QFileSystemWatcher but this costs some resources (although when only done once in the D-Bus service backend probably allright) and makes Disman again a bit more dependent on Qt.

Overall the question is if the above cases can not be ignored.