An upgrade planner.
This prototype inherits all the properties from Prototype/SelectionTool.
Can't be > 255.
show_in_library defaults to true instead of false.
show_in_library is unused as of 0.18.13 as the ability to show any selection tools in the blueprint library was removed.
Upgrade items must have a stack size of 1.
Prototype/UpgradeItem ignores the following properties that it inherits from Prototype/SelectionTool:
- selection_mode (forced to be "upgrade")
- alt_selection_mode (forced to be "cancel-upgrade")
- always_include_tiles (forced to be false)