spearmint: Add new icon for Spearmint

Square with rounded corners split into four sections, green on opposite
sections. Sort of looks like mint leaves and mint candy.

There is a rocket icon (from Turtle Arena) in the top right corner, a
reference to Q3. It was going to have an icon in each of the four
sections, but I haven't decided what else to put on it.

Internal icon is 32x32.

OS X icon has 16, 16@2x, 32, 32@2x, 128, 128@2x, 256, 256@2x, 512, 512@2x.
The '@2x' icons are double resolution.

Windows icon has 8, 10, 14, 16, 22, 24, 32, 40, 48, 64, 96, 128, 256.
Windows icons < 256 are included as 32/8/4 bit. The 256 icon is 32 bit PNG.

The 256/512 icons have "Spearmint" text using M+ 1mn Medium font.
spearmint: Update OS X app copyright