New feature: optionally hiding the menubar

[fpemud]

I think the menus on the menubar are not absolutely necessary:

1. File: we have "new game" button already.
2. Settings->Labeling: Most people play the default 2048 label.
3. Settings->Language: People won't touch it, it should be auto-selected by locale configuration.
4. Help: not very important.

so hiding the menubar won't do any harm and can lead to a clean UI.

[raspher]

If you want to get it merged, then language auto-selection by locale configuration should work. Is it working in your PR ?