fixes #124

Update to mtxclient 0.3.0

The notification spec handles -1 and 0 as special values:
-1 -> representation for "undef"
 0 -> infinitely
 values above 0 represent the timeout in ms.

The notification messages should disappear again and with 0, this insn't
the case. Setting it to -1 lets the notification manager decide the
timeout and displays the notification like all others.

The current notification spec says image-data should get used instead of
image_data.

* Working D-Bus desktop notifications

* Remove return type on constructor

* Fix the Windows placeholder class

* Fix wrong variable name

* Fix windows and macOS versions of notificationsmanager