Refactoring: Field Item Editor + buildings/terrain

Extracts logic from the building & terrain & field item editors for reuse / separation of concerns

Have FieldItemEditor select between Items & Terrain so that you can change the popup menu mode (view set delete)

Not sure how I want to show the terrain names...

NHSE.WinForms/Subforms/Map/BuildingHelp.cs

@@ -0,0 +1,32 @@
+using System;
+using System.Collections.Generic;
+using System.Windows.Forms;
+using NHSE.Core;
+
+namespace NHSE.WinForms
+{
+    public partial class BuildingHelp : Form
+    {
+        private static readonly IReadOnlyDictionary<string, string[]> HelpDictionary = StructureUtil.GetStructureHelpList();
+
+        public BuildingHelp()
+        {
+            InitializeComponent();
+            this.TranslateInterface(GameInfo.CurrentLanguage);
+
+            foreach (var entry in HelpDictionary)
+                CB_StructureType.Items.Add(entry.Key);
+            CB_StructureType.SelectedIndex = 0;
+        }
+
+        private void CB_StructureType_SelectedIndexChanged(object sender, EventArgs e)
+        {
+            var name = CB_StructureType.Text;
+            var values = HelpDictionary[name];
+            CB_StructureValues.Items.Clear();
+            foreach (var item in values)
+                CB_StructureValues.Items.Add(item);
+            CB_StructureValues.SelectedIndex = 0;
+        }
+    }
+}