eframe: Add NativeOptions::persistence_path (#4423)

This allows customizing the persistence path in NativeOptions.
Previously, persistence wouldn't work with android because
directories-next doesn't support android so eframe would just fail to
find a place where it could store its config.

* Closes #4098 (android users can now specify a path that works with
android, by e.g. using app_dirs2, which supports android)

crates/eframe/src/epi.rs

@@ -150,6 +150,7 @@ pub trait App {
     /// On web the state is stored to "Local Storage".
     ///
     /// On native the path is picked using [`crate::storage_dir`].
+    /// The path can be customized via [`NativeOptions::persistence_path`].
     fn save(&mut self, _storage: &mut dyn Storage) {}
 
     /// Called once on shutdown, after [`Self::save`].
@@ -362,6 +363,10 @@ pub struct NativeOptions {
     /// Controls whether or not the native window position and size will be
     /// persisted (only if the "persistence" feature is enabled).
     pub persist_window: bool,
+
+    /// The folder where `eframe` will store the app state. If not set, eframe will get the paths
+    /// from [directories_next].
+    pub persistence_path: Option<std::path::PathBuf>,
 }
 
 #[cfg(not(target_arch = "wasm32"))]
@@ -379,6 +384,8 @@ impl Clone for NativeOptions {
             #[cfg(feature = "wgpu")]
             wgpu_options: self.wgpu_options.clone(),
 
+            persistence_path: self.persistence_path.clone(),
+
             ..*self
         }
     }
@@ -418,6 +425,8 @@ impl Default for NativeOptions {
             wgpu_options: egui_wgpu::WgpuConfiguration::default(),
 
             persist_window: true,
+
+            persistence_path: None,
         }
     }
 }