Support for scalable font and gui elements

Fix positioning of tabheader in order to be usable for scaling GUIs
WARNING: this changes position of current tabheaders, mods have to adjust!

src/guiFormSpecMenu.h

@@ -319,7 +319,6 @@ class GUIFormSpecMenu : public GUIModalMenu
 
 	typedef struct {
 		v2s32 size;
-		s32 helptext_h;
 		core::rect<s32> rect;
 		v2s32 basepos;
 		int bp_set;
@@ -381,6 +380,8 @@ class GUIFormSpecMenu : public GUIModalMenu
 		s32 time;
 	};
 	clickpos m_doubleclickdetect[2];
+
+	int m_btn_height;
 };
 
 class FormspecFormSource: public IFormSource