LibGUI: Add PasswordBox

This patch adds a PasswordBox. At the moment, it's simply a TextBox with
it's substitution code point set to '*', and the undo and redo actions
disabled.

Userland/Libraries/LibGUI/TextBox.cpp

@@ -72,4 +72,12 @@ void TextBox::add_input_to_history(String input)
     m_history_index++;
 }
 
+PasswordBox::PasswordBox()
+    : TextBox()
+{
+    set_substitution_code_point('*');
+    undo_action().set_enabled(false);
+    redo_action().set_enabled(false);
+}
+
 }