Add menu bar toggle to View menu.

Behavior:

    Clicking the setting will show or hide the main menu bar
    across all active windows.

    While hidden, tapping the ALT key will 'temporarily' show
    the menu bar.

    Tapping and releasing ALT once more will hide the menu bar
    once more.

    Navigating to a different directory will reset this state.
    Otherwise, the menu bar will remain visible until alt-tapped
    again.

This behavior does not affect Alt-shortcut combinations for bringing
up specific menus, and mimics that of other file managers regarding
hidden menu bars.
Simplify temporary menu bar logic.
Prevent menu from appearing on the desktop (oops)
Merge pull request #166 from mtwebster/hide-menu

Add menu bar toggle to View menu.