eventmanager: added partitions labels and placement options

Initially I wanted to implement the labels only, but the second row (where labels are) may appear partially beneath JWM's panel, so users most likely will need to "fine-tune" icons' positions in such case, therefore I've implemented it as well.

Here's a screenie: http://oi39.tinypic.com/2wrp5kk.jpg

- by default labels are disabled in /etc/eventmanager (LABELPARTITIONS="false")
- labels longer than 8 chars are being truncated to 7 chars + ...
- minimal value for 'start/edge gap' is 32, because ROX (even if "Keep icons within screen limits" is unchecked) refuses to place an icon beyond left/upper edge, even partially

Greetings!