[raygui] LoadIcons-, DrawIcon-function and example added

examples/gui/icons/README.md

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+This example loads a custom .rgi-file and draws the 255th icon. You can use the [rGuiIcons](https://raylibtech.itch.io/rguiicons) tool to view or create icon files.
+
+![Screenshot](./screenshot.png)

examples/gui/icons/main.go

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+package main
+
+import (
+	"github.com/gen2brain/raylib-go/raygui"
+	rl "github.com/gen2brain/raylib-go/raylib"
+)
+
+func main() {
+	rl.InitWindow(800, 600, "raylib-go - icons example")
+	defer rl.CloseWindow()
+
+	raygui.LoadIcons("default_icons_with_255.rgi", false)
+
+	for !rl.WindowShouldClose() {
+		rl.BeginDrawing()
+		rl.ClearBackground(rl.RayWhite)
+		raygui.DrawIcon(raygui.ICON_255, 100, 100, 8, rl.Gray)
+		rl.EndDrawing()
+	}
+}