API for other mods to add their buffer value above the PlayerList

Needed for SmallTime and perhaps for Foundry Tips mods.

Allows other mods to request additional height buffer depending on their size needs. The buffer is summed from all mods that require it and is applied to the CSS param. Allow mods to actively turn on/off their needs as they are attached or unattached from above the PlayerList.

Start by locating the PlayerList overflow implementation and the CSS parameter that controls its height buffer. Read how button overflow is currently calculated, then trace how attached and unattached mods are represented. Done means other mods can turn their requested buffer on or off, all active requests are summed, and the resulting value is applied above the PlayerList.

Written by the indexing model from the issue text.