Storage and frame refactor (#1418)

The purpose of this is to expose `frame.storage()` and `frame.storage_mut()` so users can save/load app state from the `App::update` function, without having to add another parameter to that function.

Changes:
* Added `Frame::storage()` and `Frame::storage_mut()`
* `App::update` now takes a `&mut Frame` rather than just `&Frame`
* `Frame` is no longer `Clone` or `Sync` (doesn't have to be since https://github.com/emilk/egui/pull/1366)

eframe/examples/image.rs

@@ -32,7 +32,7 @@ impl Default for MyApp {
 }
 
 impl eframe::App for MyApp {
-    fn update(&mut self, ctx: &egui::Context, _frame: &eframe::Frame) {
+    fn update(&mut self, ctx: &egui::Context, _frame: &mut eframe::Frame) {
         egui::CentralPanel::default().show(ctx, |ui| {
             ui.heading("This is an image:");
             self.image.show(ui);