Build menu: Show resource icons instead of text for tooltips

Despite being awful code and potentially having layout problems all over
(which cannot come as a surprise to anyone having seen tooltip code)
this actually seems to work rather well for up to four resources.

![2013-10-13 20-20-29 494984](https://f.cloud.github.com/assets/373440/1322527/9ecc4110-3434-11e3-885a-b94419a2154e.png)
![2013-10-13 20-20-26 021249](https://f.cloud.github.com/assets/373440/1322528/ac7f8128-3434-11e3-8ec7-db82f5723c29.png)
![2013-10-13 20-20-19 628527](https://f.cloud.github.com/assets/373440/1322529/b80d7fae-3434-11e3-8d5d-fd2735ba08b6.png)