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)

epaint/src/image.rs

@@ -34,8 +34,7 @@ impl ImageData {
 
     pub fn bytes_per_pixel(&self) -> usize {
         match self {
-            Self::Color(_) => 4,
-            Self::Font(_) => 4,
+            Self::Color(_) | Self::Font(_) => 4,
         }
     }
 }