Fix terrain brush popup

Closes #729
1. Dark mode now applied correctly (was previously lacking)
2. Fixed terrain brush action (brush active is higher priority than drag, fix tile fetch)
3. Fixed writing of Building/structure labels; apply after Terrain
4. Single terrain brush form allowed, center to Field Editor on launch

Somewhat related: fix Show() load for non-ShowDialog() forms to center to parent.

NHSE.WinForms/Subforms/Program/ImageFetcher.cs

@@ -31,6 +31,12 @@ public ImageFetcher()
         CheckFileStatusLabel();
     }
 
+    protected override void OnLoad(EventArgs e)
+    {
+        CenterToParent();
+        base.OnLoad(e);
+    }
+
     private static string[] LoadHosts()
     {
         var hosts = Properties.Resources.hosts_images;