[AppImageUpdate] Show update progress bar in the dock

[probonopd]

• Ubuntu: https://wiki.ubuntu.com/Unity/LauncherAPI
• KDE Plasma: "You want to use the KNotify API" (source), but it looks like they really implemented com.canonical.Unity.LauncherEntry "We at KDE believe in collaboration which means if there’s an API or library that works and is widely adopted, we’re happy to pick it up and add support for it in our software stack." (source) https://codereview.qt-project.org/#/c/33051/2/src/plugins/coreplugin/progressmanager/progressmanager_x11.cpp
• GNOME: ???

Low level DBus API: com.canonical.Unity.LauncherEntry
https://wiki.ubuntu.com/Unity/LauncherAPI#Low_level_DBus_API:_com.canonical.Unity.LauncherEntry