Refactor and enhance webview_cef plugin

[Dimoshka]

Refactor and enhance webview_cef: Linux dialogs, init options, GPU toggle, Dart readiness fixes

Summary

This PR improves stability and usability across platforms with a focus on Linux. It adds native GTK file dialogs, exposes initialization options (userAgent, cachePath, persistence flags, enableGPU), fixes a Linux frame-id mismatch, hardens shutdown, and cleans up Dart code for stronger typing and null-safety. Build tooling on Linux now tolerates missing Debug CEF bundles by falling back to Release binaries. Documentation is updated accordingly.

Key changes

Dart

• Add explicit types and null-safe callback invocations throughout the library.
• Gate JS/eval APIs on controller readiness to avoid early-call assertions.
• Coalesce high-frequency input events (hover/drag/scroll) and debounce size reporting to reduce redundant platform traffic.
• Improve listener wiring and event routing in WebviewManager.
• Tests: initialize test bindings properly and modernize mock handlers.

Native/C++

• Linux: implement native file dialogs via GTK through CEF’s dialog handler (Open/Save/Select Folder; multi-select; simple filters).
• Normalize frame-id comparisons on Linux by converting CefFrame::GetIdentifier() to a consistent int64_t before comparisons.
• Threading: ensure operations run on the correct CEF threads (UI/IO) for input, cookies, and lifecycle; add helper for posting lambdas.
• Shutdown: close all browsers on the CEF UI thread and clear references before CefShutdown to avoid atexit teardowns and fatal asserts.
• DevTools shortcut routing and lifecycle counters for safe shutdown after last browser closes.

Build/CMake (Linux)

• Add DOWNLOAD_CEF_FLAVOR option (standard | minimal) to control prebuilt size.
• If Debug CEF binaries are missing, fall back to Release libcef.so and resources to enable Debug builds of the plugin.

Initialization options

New runtime options (Dart WebviewManager.initialize → native CEF settings):

• userAgent: product string appended to the default UA.
• cachePath: writable directory for on-disk cache and preferences.
• persistSessionCookies: persist session cookies (requires non-empty cachePath).
• persistUserPreferences: persist user preferences (requires non-empty cachePath).
• enableGPU: toggle GPU acceleration (applies command-line switches accordingly).

Documentation

• README: document Linux native file dialogs, Debug→Release fallback, CEF flavor option, and initialization options with examples and notes.

Cross-platform notes

• cachePath, persistSessionCookies, and enableGPU are applied across platforms (subject to CEF constraints).
• persistUserPreferences may depend on CEF build exposure; if unavailable on a platform, it’s ignored safely.
• Linux dialogs are native GTK; no extra Flutter plugin is required.

Verification

• Dart analyzer: clean (no missing type annotations; null-safety observed).
• Linux: example builds and runs; dialogs functional; early JS/eval no longer assert; Debug builds succeed with Release CEF fallback.
• Manual QA: navigation, console messages, devtools open, pointer/scroll input, IME composition updates rendered and routed to Flutter.

Breaking changes

• None expected. The new init options are optional and default to previous behavior when omitted.

Migration

• Optional: call WebviewManager().initialize({...}) early in app startup if you want custom UA, persistent cache/cookies, or to enable GPU.

Next steps (optional)

• Consider exposing a Flutter-side file picker override for apps that prefer a custom UI.
• Stress-test shutdown across repeated create/close cycles on Windows and macOS.