Dynamic water (plane) level

Bill of materials to be done before starting this ticket:
* #1445

----

It would be good if the water plane height could be adjusted dynamically. This is a pretty common request but has historically been considered difficult, both to implement in engine and to workaround game-side. Currently to raise the water level by X you have to adjust the entire terrain by -X instead, which comes with its own caveats.

A water plane level getter function exists but is not used comprehensively (see #1445). The core of this ticket is a request is to pair it up with a setter, initially just for the plane, and of course expose it to Lua.

Start with prerequisite issue #1445 and trace the existing water plane level getter through the engine and its Lua exposure. The work is complete when a corresponding setter can dynamically adjust the water plane height and is available to Lua.

Written by the indexing model from the issue text.