show number of units selected, instead of a bunch of the same icons

[andy5995]

@Jammyjamjamman suggested this in #28

My suggestion for how to display what's selected: show an icon of the unit, along with the number selected.

e.g. "icon1 X 14, icon2 X 33... etc"

Or the number could be overlaid onto the icon (idea pinched from 0 A.D.)

In this example, there are 4 elf icons in the upper-right corner.