Fixes positioning of icons across multiple screens for desktop.lua

[rjhwelsh]

Icons were behaving strangely across multiple monitors, although I noticed it worked on the one at 0,0

Seems there was a mismatch in global positioning. I fixed the mismatch.

Now they work across my all my screens. :D

Additionally:

• Not sure what variable "totheight" is named after, but it was giving me temporary dyslexia so I renamed it to "tot_height".
• I re-adjusted the 40 hard-coded margin to 20 to account for adding in an extra margin using the margin.y parameter.