Add condition for XDG_CURRENT_DESKTOP checks
Add condition for XDG_CURRENT_DESKTOP checks

Firstly this changes the "download-if" and "enable-if" properties
to accept a `;` separated list of multiple conditions.

Secondly it adds `on-xdg-desktop-*` which will check against
the XDG_CURRENT_DESKTOP env var (case-insensitively).

This is done entirely for the Qt GNOME Platform so it can do this:

```
"org.kde.PlatformTheme.QGnomePlugin" : {
  "download-if": "on-xdg-desktop-GNOME;on-xdg-desktop-GNOME-classic"
}
```

Closes: #1436
Approved by: matthiasclasen

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