views: Draw underline in menu bar for accelerators.
views: Show menubar's accelerator when "Alt" is pressed.
views: Show menubar's submenu when "Alt+Key" is pressed.
views: Pressing "Alt+Key" should bring up the menu bar.
views: Always hide accelerator when submenu is activated.
views: Fix calculating underline's position.
win: Underline's color tends to be a little lighter.
views: Fix activating lower case accelerators.
A little cleanup.
Merge pull request #585 from atom/menubar_underline

Support specifying accelerator for menu bar items by adding "&" in label