Hydroponics fixes, icons and irrigation

Syringes will switch to draw mode if emptied into plots.
All reagent containers have icon updated after plot-use, including droppers.
Droppers updated to use update_icon instead of that "filled" variable.

The above fix issue #3879

Typo in irrigation fixed, allowing liquid > 30 units to be spread amongst connected trays.
Changelog updated, including some things from previous PRs which I didn't add at the time.