LibGUI+WindowServer: Add "visible" state to GUI actions

This patch adds a visibility state to GUI::Action. All actions default
to being visible. When invisible, they do not show up in toolbars on
menus (and importantly, they don't occupy any space).

This can be used to hide/show context-sensitive actions dynamically
without rebuilding menus and toolbars.

Thanks to Tim Slater for assuming that action visibility was a thing,
which gave me a reason to implement it! :^)

Userland/Libraries/LibGUI/Button.cpp

@@ -165,6 +165,7 @@ void Button::set_action(Action& action)
 {
     m_action = action;
     action.register_button({}, *this);
+    set_visible(action.is_visible());
     set_enabled(action.is_enabled());
     set_checkable(action.is_checkable());
     if (action.is_checkable())