Chemdispenser Overlays [feature]

Requested by Paprka in #6161 .
Adds a simple overlay to the chem dispensers to show you if there's a beaker in it
![minidisp](https://cloud.githubusercontent.com/assets/7305553/5268610/cfd7c756-7a52-11e4-8ce0-5a4aab2c8e26.png)



Commits 1&2 are failed implementations that were removed, but due to -gitmagic- won't go away, no matter if I switch branches or not.
Yes, it is le early, but the codefreeze has petered out and I finally made this work nicely for both.

You should use overlays.Cut() instead, it's faster.