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/dropdowns.yaml

@@ -26,6 +26,36 @@ ScrollPanel@LABEL_DROPDOWN_TEMPLATE:
 					Width: PARENT_WIDTH - 20
 					Height: 25
 
+ScrollPanel@LABEL_DROPDOWN_WITH_TOOLTIP_TEMPLATE:
+	Width: DROPDOWN_WIDTH
+	Children:
+		ScrollItem@HEADER:
+			Background: scrollheader
+			Width: PARENT_WIDTH - 27
+			Height: 13
+			X: 2
+			Y: 0
+			Visible: false
+			Children:
+				Label@LABEL:
+					Font: TinyBold
+					Width: PARENT_WIDTH
+					Height: 13
+					Align: Center
+		ScrollItem@TEMPLATE:
+			Width: PARENT_WIDTH - 27
+			Height: 25
+			X: 2
+			Y: 0
+			TooltipContainer: TOOLTIP_CONTAINER
+			TooltipTemplate: BUTTON_TOOLTIP
+			Visible: false
+			Children:
+				Label@LABEL:
+					X: 10
+					Width: PARENT_WIDTH - 20
+					Height: 25
+
 ScrollPanel@PLAYERACTION_DROPDOWN_TEMPLATE:
 	Width: DROPDOWN_WIDTH
 	Children: