Build menu: Show resource icons instead of text for tooltips

Despite being awful code and potentially having layout problems
(which cannot come as a surprise to anyone having seen tooltip code)
this actually seems to work rather well for up to four resources.

The only currently available building causing slight trouble is our
wooden tower.
Fix tooltip icons not scaling with Fife 0.3.5 or older
Merge pull request #2171 from unknown-horizons/icon-tooltip

Build menu: Show resource icons instead of text for tooltips