KeyStores with unavailable entries

I've changed the meaning of entry availability.  It used to
be that an entry 
is available if the store is available.  Now, an entry is
available if its 
private content is also available, and this value is
controlled by the 
provider, via the new isAvailable() virtual function in
KeyStoreEntryContext 
(default returns true).  Otherwise, no new methods were
added.  
KeyStoreEntry::isAvailable() and ensureAvailable() and
KeyStoreEntryWatcher 
available/unavailable signals are all still there, but now
with just a 
slightly different meaning.

It is therefore now possible to have entries that are
unavailable, even though 
its store is available.  For example, this can happen:

  KeyStoreEntry e = keyStore.entryList().first();
  if(!e.isAvailable())
    printf("entry is in the store but not available for
private usen");

To indicate an availability change, the provider uses
storeUpdated, which 
indicates if any entries were added, removed, or modified. 
Availability is 
considered a modification.

What I'm unsure about is how to handle prompting.  If an
entry is unavailable 
(but its store is available), what should the user be
prompted for?  If you 
prompt for the store that the entry is in, then that doesn't
really solve 
anything since the store is already present.

For example, try this with qca-test and qcatool:

  echo hello world > hello.txt
  qcatool2 keystore exportref test:test > fake.ref
  qcatool2 message sign smime fake.ref < hello.txt

Currently, the Test Store 1 will appear after 2 seconds,
containing an 
unavailable Test Cert 1 entry.  After 4 more seconds, Test
Cert 1 is made 
available.  What you see qcatool do is auto-accept the token
prompt after 2 
seconds (because it sees the store appear), and then use
100% cpu for the 
next 4 seconds as the provider keeps token prompting (entry
not available 
yet) and qcatool keeps auto-accepting.

One possible solution is for the keyStoreEntry field of the
token event to 
indicate that a particular entry is being prompted for, and
that the 
application should not auto-accept until that exact entry is
available.

I tried this in my local copy of qcatool (patch attached). 
I have it use the 
KeyStore updated() signal to check for an availability
change on the prompted 
entry before auto-accepting.  If the keyStoreEntry field is
null, then it 
will auto-accept if just the store is available.  This works
great.  Now 
qcatool sleeps 6 seconds and then auto-accepts.

This may be plenty, but I have to figure out what prompt
language to use.  
Right now it says "Please insert the 'Test Store 1'
token for Test Cert 1 and 
press Enter...", but that's silly because the Test
Store 1 is already 
present.  Maybe it should say "Please make Test Cert 1
available (of Test 
Store 1) and press Enter...".

Thoughts?

-Justin

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