Prune old egui memory data when reaching some limit

[emilk]

• Closes Add a way to clear storage #2641

Previously egui would just accumulate state over time, growing its memory more and more if the user had unique Ids with persisted state, e.g. a lot of CollapsingHeaders or Windows with different names. This leads to slow load and save of eframe apps with persistence.

With this PR egui will now forget the oldest data of each type that hasn't been read in a while. That is, if you load and save your app state many times, egui will eventually drop the thing that never been read.

The limit can be set with egui_ctx.memory_mut(|m| m.data.set_max_bytes_per_type(1_000_000));

The default is 256 kiB per type. For reference, a CollapsingHeader takes around 200 bytes to store, so that's over a thousand different collapsing headers in your application.