NullPointerException on systems without `gnome-keyring` installed or `login` as default collection

[swiesend]

NullPointerException on systems without `gnome-keyring` installed or `login` as default collection

[swiesend]

The MessageHandler is updated and does not consume connection problems. Instead it throws any occurring exception as DBusException. This change led to an unfortunate big commit, but was inevitable in order to not commit non compiling code. I ran the test on my machine (Ubuntu 18.04 GNOME), but still have to test how the library behaves on other systems with and without gnome-keyring installed. I guess I should provide a small sample application or build Cryptomator with the current changes.

Now the high-level SimpleCollection API consumes and logs occuring DBusExceptions and explains connection problems by throwing IOExceptions with a short message.

Most returning methods of the SimpleCollection return now an Optional, except of getLabel() and getSecret(). If there is no security concern to wrap the char[] of getSecret() as Optional, then it may be more consistent to let all returning functions return an Optional.