Allow to run 'global webapps'

This allows to put the desktop file to /usr/share/applications and use
an icon from e.g. /usr/share/icons.

Tested with this desktop file:

```
[Desktop Entry]
Name=Webapp test
Exec=epiphany --webapp-desktop-file=/usr/share/applications/epiphany-webapp-test.desktop --application-mode --profile="/home/purism/.local/share/epiphany-webapp-test" https://social.librem.one/@agx
StartupNotify=true
Terminal=false
Type=Application
Categories=Network;GNOME;GTK;
Icon=/tmp/app-icon.png
StartupWMClass=webapp-test
```

The profile directory does not need to exist. Webapps still need
to start with `epiphany-`.
3 jobs for global-webapp in 0 seconds (queued for 85 minutes and 21 seconds)
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