Support for persistent TouchID unlock.

While originally not intended, this changeset enables MacPass to
unlock a database with TouchID even after the process is completly
wiped.

It does this by introducing multiple modes of operation.

First:  TouchId can be completly disabled. The TouchID checkbox is off
        and MacPass works like the TouchID feature had never been added.

Second: The TouchID checkbox gets put into the mixed state. MacPass will now
        remember the database key in memory as long as the process remains
	alive and the database can be unlocked with TouchID until the
	applications terminates.

Third: The TouchID checkbox is checked and MacPass will store the encrypted
       database key on a successfull unlock attempt in the standard
       userdefaults. TouchID unlock works now even after MacPass is completly
       terminated and restarted.

MacPass/MPSettingsHelper.m

@@ -63,6 +63,7 @@ NSString *const kMPSettingsKeyAutotypeMatchTags                           = @"Au
 NSString *const kMPSettingsKeyGloablAutotypeAlwaysShowCandidateSelection  = @"GloablAutotypeAlwaysShowCandidateSelection";
 
 NSString *const kMPSettingsKeyEntryTouchIdEnabled                     = @"EnableSubsequentUnlocksWithTouchID";
+NSString *const kMPSettingsKeyEntryTouchIdDatabaseEncryptedKeyFormat  = @"EncryptedDatabaseKeyForTouchID-%@";
 
 NSString *const kMPSettingsKeyEntrySearchFilterContext                    = @"EntrySearchFilterContext";