Create generic GWIN Draw routine

For some GWIN controls they support a Draw() routine.
For those controls the base class now supports a generic Draw routine
which will call their specific Draw routine.
The only GWIN's with a Draw routine currently are Buttons and Sliders.

include/gwin/gwin.h

@@ -165,6 +165,16 @@ void gwinDestroyWindow(GHandle gh);
 
 /* Drawing Functions */
 
+/**
+ * @brief   Draw the window
+ * @note	Redraws the Window if the GWIN object has a draw routine
+ *
+ * @param[in] gh		The window handle
+ *
+ * @api
+ */
+void gwinDraw(GHandle gh);
+
 /**
  * @brief   Clear the window
  * @note	Uses the current background color to clear the window

src/gwin/gwin.c

@@ -107,6 +107,21 @@ void gwinDestroyWindow(GHandle gh) {
 	}
 }
 
+void gwinDraw(GHandle gh) {
+	switch(gh->type) {
+	#if GWIN_NEED_BUTTON
+		case GW_BUTTON:
+			gwinButtonDraw(gh);
+			break;
+	#endif
+	#if GWIN_NEED_SLIDER
+		case GW_SLIDER:
+			gwinSliderDraw(gh);
+			break;
+	#endif
+	}
+}
+
 #if GDISP_NEED_TEXT
 	void gwinSetFont(GHandle gh, font_t font) {
 		gh->font = font;