LibGUI: Add allowed file extensions to FileSystemModel

This allows FileSystemModel to take an optional list of allowed file
extensions which it will use to filter out all files that don't end
with that file extension.

The file extensions are set via `set_allowed_file_extensions` which has
a coresponding `get_allowed_file_extensions`.

Userland/Libraries/LibGUI/FileSystemModel.h

@@ -146,6 +146,9 @@ public:
     bool should_show_dotfiles() const { return m_should_show_dotfiles; }
     void set_should_show_dotfiles(bool);
 
+    Optional<Vector<DeprecatedString>> allowed_file_extensions() const { return m_allowed_file_extensions; }
+    void set_allowed_file_extensions(Optional<Vector<DeprecatedString>> const& allowed_file_extensions);
+
 private:
     FileSystemModel(DeprecatedString root_path, Mode);
 
@@ -169,6 +172,8 @@ private:
     unsigned m_thumbnail_progress { 0 };
     unsigned m_thumbnail_progress_total { 0 };
 
+    Optional<Vector<DeprecatedString>> m_allowed_file_extensions;
+
     bool m_should_show_dotfiles { false };
 
     RefPtr<Core::FileWatcher> m_file_watcher;