Move file association handling back into the app

The DeployMaster installer issue prevents the user from seeing more
than nine of the 18 file extensions that CiderPress wants to handle,
and I don't want to go stomping on file associations without some
way to disable the behavior.  So this returns to the previous behavior,
where CiderPress directly manages the file associations.

The CiderPress app is not able to modify HKEY_LOCAL_MACHINE (which
it used to do via HKEY_CLASSES_ROOT) on recent versions of Windows --
tested in Win7, but it probably broke with Vista.  So now we do
everything in HKEY_CURRENT_USER.  This works, more or less.

We're not looking at the Windows shell overrides, which are made
in yet another set of registry entries, so there are multiple
reasons why the values reported by the Edit Associations dialog may
now be inaccurate.  I still favor eliminating the dialog as a
long-term strategy.

I took the opportunity to do some code cleanup in the registry code.
I also added calls to SHChangeNotify() to tell the Windows shell when
file associations change, so Windows Explorer windows get updated
promptly.