Added "quit" menu item, keyboard shortcuts for Open,Quit.
Toggle left-side bar, improve View menu items.

Make menu items "checkable",
rename them to be more informative.
Create "Window" menu

Move menu items from "View" to "Window",
Add Cascade, Maximize, Restore, Minimize.
New option: show MDI windows as Tabs.
UI: Added menu item shortcuts (ALT+letter).
Merge pull request #6 from agordon/keyboard_shortcuts

Keyboard and UI improvements