Enhancement: Built-in AppImage self-updater

In release 2.10, we introduced a preview on our future AppImage packaging for Linux distributions.
Now, these AppImages can self-update using a built-in `libappimageupdate` based updater and
ownCloud's update infrastructure.

https://github.com/owncloud/client/issues/8923
https://github.com/owncloud/client/pull/9376
