LibGUI: Add transient option to show dotfiles in FilePicker

This is particularly useful when wanting to open files in ~/.config
from the Text Editor. The option is currently not persistent, but could
be hooked into File Manager's configuration.

Userland/Libraries/LibGUI/FilePicker.h

@@ -83,6 +83,7 @@ private:
 
     RefPtr<TextBox> m_filename_textbox;
     RefPtr<TextBox> m_location_textbox;
+    RefPtr<Menu> m_context_menu;
     Mode m_mode { Mode::Open };
 };