Add mutliplier newline_spacing

doc/lua_api.txt

@@ -2754,8 +2754,9 @@ Some types may inherit styles from parent types.
             * `label`: Bottom of first line
             * `vertlabel`: Bottom of first character
       Default `centerline` for label and `top` for vertlabel.
-    * newline_spacing - number/`auto`, sets space in coordinates between lines/columns of text.
-      If set to `auto`, the text is as close together as possible without overlap. Default 0.5.
+    * newline_spacing - number, sets space between lines/columns of text. If
+      the number is prefixed with `*`, the spacing is set to a multiple of the
+      label's font height or width, otherwise it's in coordinates. Default `0.5`.
 * image_button (additional properties)
     * fgimg - standard image. Defaults to none.
     * fgimg_hovered - image when hovered. Defaults to fgimg when not provided.

src/gui/guiFormSpecMenu.cpp

@@ -1806,10 +1806,12 @@ void GUIFormSpecMenu::parseLabel(parserData* data, const std::string &element)
 
 		std::string str_newline = style.get(StyleSpec::NEWLINE_SPACING, "0.5");
 		float newline_spacing;
-		if (trim(str_newline) == "auto")
-			newline_spacing = (float)font_height;
-		else
+		if (str_newline.size() > 0 && str_newline[0] == '*') {
+			str_newline = str_newline.substr(1);
+			newline_spacing = stof(str_newline) * (float)font_height;
+		} else {
 			newline_spacing = stof(str_newline) * (float)imgsize.Y;
+		}
 
 		s32 rect_height = abs(newline_spacing) * (float)(lines.size() - 1) + font_height;
 
@@ -1989,8 +1991,10 @@ void GUIFormSpecMenu::parseVertLabel(parserData* data, const std::string &elemen
 
 		std::string str_newline = style.get(StyleSpec::NEWLINE_SPACING, "0.5");
 		float newline_spacing = 0;
-		if (trim(str_newline) == "auto") {
-			newline_spacing = (float)font_width + m_font->getKerningWidth();
+		if (str_newline.size() > 0 && str_newline[0] == '*') {
+			str_newline = str_newline.substr(1);
+			newline_spacing = stof(str_newline) * (float)font_width +
+				m_font->getKerningWidth();
 		} else {
 			newline_spacing = stof(str_newline) * (float)imgsize.X;
 		}