Add tooltips to some map generator settings

Adds tooltip descriptions for a few map generator settings. The tooltips
are limited to dropdown choices. No tooltips are added for the options
containing the choices or any non-dropdown settings.

- Terrain type
- Bounds shape
- Buildings

The "Square" bounds shape is rephrased as "Rectangle" for consistency
with the added description.

This is UI-only change.

mods/common/chrome/editor.yaml

@@ -621,6 +621,7 @@ Container@EDITOR_WORLD_ROOT:
 											Y: 20
 											Width: PARENT_WIDTH
 											Height: 25
+											PanelRoot: EDITOR_WORLD_ROOT
 		Container@HISTORY_WIDGETS:
 			X: WINDOW_WIDTH - 320
 			Y: 354