[New Feature] UI tooltip for Harvester counter

[Damfoos]

Would be nice to have a tooltip pop up when you hover the mouse over the harvester counter, as most of the interactive elements of the sidebar have such tooltips already. This will be useful for players who are new to phobos-based mods and are not used to what this thing is.

Something like a csf tag UI:HarvCounter, so the modders can customize it. If the string is not present in a stringtable, display a text by default. The text could be something simple like "Harvester counter" or "Harvesters mining/total"