* Rename epaint feature "persistence" to "serialize"
* Add separate "serialize" feature to egui
* egui_demo_lib: separate serialize and persistence features
* Add App::persist_native_window and App::persist_egui_memory
Controls what gets persisted
Label text will now be centered, right-aligned and/or
justified based on the layout.
Galleys are no longer always pivoted in the left top corner,
so now have a Rect rather than just a size.