Added installdesktop plugin

[trumpton]

This plugin adds a hook script which is run before the main application in the AppImage.
The hook script intercepts an --install-desktop switch, and if provided, converts / parses the .desktop file and installs it into either the system folder or the user's .local folder. It also copies the application icon across.

The resulting effect is "application.AppImage --install-desktop" puts the appropriate files in place so that the application appears in the linux start menu.

This has been tested on Ubuntu 22 and Ubuntu 20 with plasma desktop.

[TheAssassin]

I doubt that this is incredibly useful. The AppImage community has had bad experiences with including desktop integration features within AppImages.

Does this implement a libappimage compatible algorithm?

[trumpton]

You are probably right - I've been frustrated as for a non-technical user, getting things added to the start menu is complex. It's not like having an 'install.exe' which puts the programs, data files and menu files in place.

The script is a simple bash one, and doesn't use any third party libraries.

Looking at go-appimage / appimaged, it looks like that achieves what is needed. Shame that is not available as part of a standard distribution (at least on Ubuntu), so the user still needs to be quite technical.

Propose closure of this PR.

[TheAssassin]

Please also check out AppImageLauncher, which is shipped as part of the default distro on KDE Neon and a few others.

[TheAssassin]

For the record, this is documented in https://docs.appimage.org/user-guide/run-appimages.html#integrating-appimages-into-the-desktop.