Nicer way to enter path to put the collection

Originally reported on Google Code with ID 2217

```
In the prefs, you can specify the path where AD stores the collection, e.g. to put it
on an external SD-card. Currently you have to type in the path via keyboard.

It would be nice to use some kind of GUI directory picker.

The way to go seems to be to launch the user’s preferred file browser, if there is
one, and to add a simple file browser to AD as a fallback.
Maybe this one
https://github.com/iPaulPro/aFileChooser
could be used for that. Maybe there are nicer ones out there.

Is this feature available on similar apps (Anki Desktop, AnkiMobile,
AnyMemo, etc) ? If yes, it could help to include a screenshot or link to a
page explaining the feature.

Anki desktop is using standard OS file dialogs all the time…
```

Reported by `ospalh` on 2014-07-31 09:43:47