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 three resources.

Still need to figure out a way of dealing with 4+ required resources
like, for instance, our tower and many later buildings (bricks).

And, of course, a more sophisticated system would enable custom tooltip
designs based on some kind of widget subsystem. Last time I tried coding
up such a system, the logbook code ended up all messy and contaminated.
You're most welcome to take the good parts of that system and make
tooltip code work with it, though. Just remember to keep your sanity
along the way :-)