examples: add a gg drag'n'drop example (#11403)

examples/gg/drag_n_drop.v

@@ -0,0 +1,70 @@
+module main
+
+import os
+import gg
+import gx
+import sokol.sapp
+
+const (
+	max_files = 12
+	text      = 'Drag&Drop here max $max_files files.'
+	text_size = 16
+)
+
+struct App {
+mut:
+	gg                &gg.Context
+	dropped_file_list []string = []string{}
+}
+
+fn main() {
+	mut font_path := os.resource_abs_path(os.join_path('../assets/fonts/', 'RobotoMono-Regular.ttf'))
+	mut app := &App{
+		gg: 0
+	}
+	app.gg = gg.new_context(
+		bg_color: gx.rgb(174, 198, 255)
+		width: 600
+		height: 400
+		window_title: 'Drag and drop'
+		frame_fn: frame
+		font_path: font_path
+		user_data: app
+		event_fn: my_event_manager
+		// drag & drop
+		enable_dragndrop: true
+		max_dropped_files: max_files
+		max_dropped_file_path_length: 2048
+	)
+	app.gg.run()
+}
+
+fn my_event_manager(mut ev gg.Event, mut app App) {
+	// drag&drop event
+	if ev.typ == .files_droped {
+		num_dropped := sapp.get_num_dropped_files()
+		app.dropped_file_list.clear()
+		for i in 0 .. num_dropped {
+			app.dropped_file_list << sapp.get_dropped_file_path(i)
+		}
+	}
+}
+
+fn frame(mut app App) {
+	app.gg.begin()
+
+	mut txt_conf := gx.TextCfg{
+		color: gx.black
+		align: .left
+		size: int(text_size * app.gg.scale + 0.5)
+	}
+	app.gg.draw_text(12, 12, text, txt_conf)
+
+	mut y := 40
+	for c, f in app.dropped_file_list {
+		app.gg.draw_text(12, y, '[$c] $f', txt_conf)
+		y += text_size
+	}
+
+	app.gg.end()
+}