kanji grid: show waiting progress bar while the new kanji grid contents are recomputed.

```
Currently, the user cannot see whether the kanji grid has already been comluted 
and updated, or whether it is still being updated (or is not updated at all, 
because of an error)

```

Original issue reported on code.google.com by `martin.v...@gmail.com` on 20 Dec 2014 at 10:40
