House picker: Allow only one instance, remove button toggle behaviour.

This makes it behave more like object picker window.

src/terraform_gui.cpp

@@ -614,10 +614,7 @@ struct ScenarioEditorLandscapeGenerationWindow : Window {
 				break;
 
 			case WID_ETT_PLACE_HOUSE: // Place house button
-				if (HandlePlacePushButton(this, WID_ETT_PLACE_HOUSE, SPR_CURSOR_TOWN, HT_RECT)) {
 				ShowBuildHousePicker(this);
-					this->last_user_action = widget;
-				}
 				break;
 
 			case WID_ETT_INCREASE_SIZE:

src/town_gui.cpp

@@ -1717,6 +1717,9 @@ static WindowDesc _house_picker_desc(
  */
 void ShowBuildHousePicker(Window *parent)
 {
+	if (BringWindowToFrontById(WC_BUILD_HOUSE, 0) != NULL) {
+		return;
+	}
 	new HousePickerWindow(&_house_picker_desc, parent);
 }