Add keyboard shortcut reference card overlay

Creates a "quick reference" keyboard shortcut card that can be displayed from the Help menu, or via `Alt+Shift+K`. 

The card is built from the existing keyboard shortcut information in `Commands.cmd.xml`, with two additions:

- The shortcuts are now organized into groups (`<shortcutgroup>`). This has no effect on their runtime operation but permits the shortcuts to be declaratively separated.
- There is a new type of shortcut: unbound shortcuts. These are distinguished from regular shortcuts by the the lack of a bound command `refid`, and exist only for documentation of shortcuts that are handled outside of the shortcut manager (e.g. ACE shortcuts). 

When the reference card is invoked, the shortcuts are sorted by group and displayed in a translucent overlay, which is dismissed after being acknowledged by any keypress or mouse click. 

This doesn't replace the existing keyboard shortcut docs, since not all the keyboard shortcuts that exist would fit on a screen, and some of the situations in which shortcuts can be used and/or instructions for invoking them are too complex to summarize in the reference card.