cannot be compiled without egui_persistence when egui is compiled with persistence

[XertroV]

this isn't too much of an issue i guess because not much is persisted, however, it does make #164 harder to debug. (I'm using the 'Reset egui' button from the demo backend panel to fix it; but not everyone will have that)

[blitzarx1]

Before going stable we will decide what goes into the default feature set. For the moment if you want to use persistence feature in egui you need to explicitly enable egui_persistence feature