Create Jitsi Meet app

* Install Chromium
* Install Jitsi Meet extension in Chromium
* Run `desktop2app ~/Desktop/chrome-ibiognfelmneebngbnbeonnllapmffmb-Default.desktop ~/Desktop`

This is the result:


Unfortunately it did not carry over the arguments from the desktop file. Should be `/usr/local/share/chromium/chrome --profile-directory=Default --app-id=ibiognfelmneebngbnbeon
Icon=chrome-ibiognfelmneebngbnbeonnllapmffmb-Default`.

Need to work on `desktop2app` to carry over the arguments.

Need to work on `/usr/local/libexec/app-placeholder` to use them. (Maybe rename it, since it now is the same code as the Update utility.)

Possible enhancements:
Make it use its own profile directory? So that cookies etc. won't be shared outside of this app.

Fine-tune that placeholder AppDir. Once satisfactory, upload into repository.