epi: drag native window (#728)

* feat: drag window

* Update epi/src/lib.rs

Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

* Update egui_demo_lib/src/backend_panel.rs

Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

* cargo fmt

Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

egui_web/src/backend.rs

@@ -259,6 +259,7 @@ impl AppRunner {
                 quit: _,        // Can't quit a web page
                 window_size: _, // Can't resize a web page
                 decorated: _,   // Can't show decorations
+                drag_window: _, // Can't be dragged
             } = app_output;
         }