[PATCH] Tribute enhancements

[alpha123]

A collection of miscellaneous small improvements to tributing.

tribute-notifications.patch: notifies the receiver of a tribute, "alpha123 has sent you 1000 food."

tribute-insufficient-resources.patch: notifies the sender of a tribute if they have insufficient resources. Actually, this patch is so trivial it removes more code than it adds. :P

tribute-shift-click.patch: allows for tributing to function a bit like unit training. Shift-click will tribute 500, shift-click-click will tribute 1000, etc. Also changes the tooltip for the tribute resource icons to be more informative, "Tribute 100 food to alpha123. Shift-click to tribute 500."

None of these patches depend on each other, so apply as many as you like.