Fix incorrect refreshing of pick-up caption

End of first line in the pick-up menu isn't properly erased, and this can causes some artifacts.
Dynamical definition of width of pick-up menu allows to erase everything during refreshing.

How to reproduce the fixed bug:
1) make your terminal window wide enough for using standard sidebar;
2) collect a lot of items (Vol. 99/126 is fine);
3) stay on tile with pile of items and activate pick up menu by pressing 'g';
4) select a few items for making the record of your carried volume longer, something like 100/126 is okay;
5) de-select these items and get the record 'Vol 99/1266'.
![inv_vol](https://f.cloud.github.com/assets/5421954/2326341/ba9372b6-a3e1-11e3-957a-1c75faf7b87f.png)