FullscreenUI: Use native file selector on Flatpak

src/util/imgui_fullscreen.h

@@ -314,3 +314,16 @@ void GetChoiceDialogHelpText(SmallStringBase& dest);
 void GetFileSelectorHelpText(SmallStringBase& dest);
 void GetInputDialogHelpText(SmallStringBase& dest);
 } // namespace ImGuiFullscreen
+
+// Host UI triggers from Big Picture mode.
+namespace Host {
+/// Returns true if native file dialogs should be preferred over Big Picture.
+bool ShouldPreferHostFileSelector();
+
+/// Opens a file selector dialog.
+using FileSelectorCallback = std::function<void(const std::string& path)>;
+using FileSelectorFilters = std::vector<std::string>;
+void OpenHostFileSelectorAsync(std::string_view title, bool select_directory, FileSelectorCallback callback,
+                               FileSelectorFilters filters = FileSelectorFilters(),
+                               std::string_view initial_directory = std::string_view());
+} // namespace Host