FIX GncXML export

Slots before the fix:
```
<slot>
    <slot:key>placeholder</slot:key>
    <slot:value type="string">false</slot:value>
    <slot:key>favorite</slot:key>
    <slot:value type="string">false</slot:value>
</slot>
```
Which should be:
```
<slot>
    <slot:key>placeholder</slot:key>
    <slot:value type="string">false</slot:value>
</slot>
<slot>
    <slot:key>favorite</slot:key>
    <slot:value type="string">false</slot:value>
</slot>
```

Don't affect import to App.
Merge pull request #213 from fefe982/dev-xml-export-fix

FIX slot tags in GncXML export