[FIX] Properly handle invalid values for passphrase cache time [#69 state:fixed]

- Previously, if any value was entered into the passphrase cache time text field that is not a number
  or a number which is bigger than NSInteger GPGPreferences wreaks havoc.
- Handle this case gracefully and simply display the default value again.
[FIX] Display placholder for "default key" if no key is selected [#70 state:fixed]