Sometimes (it looks like maybe when the Chocolatey Licensed Extension is
in play) we don't get the sources currently in the configuration file.
If instead of reusing the configuration settings service we instantiate
a new one each time, then it will ensure the configuration file is read
again. This resolves the issue of settings not applying in GUI if
they're made while GUI is running.

When Chocolatey CLI makes changes to the configuration file while
Chocolatey GUI is running, Chocolatey GUI should refresh the sources so
that they're accurate. This adds a FileSystemWatcher to monitor the
configuration file and update the GUI if there's updates to the file.

With the FileSystemWatcher, we don't need to manually tell Chocolatey
GUI to update the sources, as they'll be updated by the
FileSystemWatcher. Leaving them in results in duplicated entries that we
don't want.

Renamed configuration file variable to indicate it's the path to the
configuration file. Made ConfigFileChanged public and added it to the
interface. Also renamed it to OnConfigFileChanged as that seems to be a
better name to indicate it's the action for when the file changes.

This reverts commit 583b1b9.