shutter: wrap `XDG_DATA_DIRS` with `hicolor-icon-theme`

When opening `shutter` it adds an indicator icon to the status bar.
However this doesn't happen (and an ugly default icon will be used) if
`shutter` can't find the `hicolor-icon-theme`. In such a case a warning
like this can be found in `stderr`:

```
Gtk-WARNING **: Could not find the icon 'image-png'. The 'hicolor' theme
was not found either, perhaps you need to install it.
```

As I don't think that we should force users to install this theme
globally and several other packages including `tor-browser`, `gparted`
or `clawsmail` add `hicolor-icon-theme` to their closure this seems to
be a fair measure.