Update support and keyboard documentation

Problem Statement
Some users are much quicker when navigating with a keyboard, instead of the mouse. While the application has many keyboard shortcuts, some improvements could be made to improve it.

Evidence or User Insights

Proposal
Here is what I'm proposing:

1. Improve the discoverability of keyboard shortcuts. Currently, pressing the 'alt' key reveals what keys will press, which is great:
   

It would be better if there was a way of displaying all the keys, including the various functions, or changing modes. For example, the F1 key (which is traditionally bound to bringing the 'help' for the application) is completely unbound; it would be useful if it showed a list of the keyboard shortcuts, or perhaps some labels near the keys similar to what happens when pressing 'Alt'.

2. Assign keys to the buttons which don't have any. For example, in the "Programmer" view, there is no key to toggle between "keypad" and "bit" modes. Also, in the "bit" mode, you can't type any number using the keyboard (you could in the old win7 calculator).

3. Extensively test keyboard navigation. Recently, I found #152 which is a pretty annoying issue for people who use the keyboard extensively.

Goals
Users who are more comfortable with using the keyboard can use Calculator without having to touch the mouse, and can do their work much more quickly.

Non-Goals

Low-Fidelity Concept
Here is a crudely drawn concept for proposal 1:

Alternatively, this is another way of implementing the proposal:

Start by reviewing the Programmer view's keyboard behavior and the related issue #152. Compare the current Alt-key hints, F1 behavior, mode switching, and numeric input against the proposed keyboard-navigation goals. Done means the agreed shortcut-discovery and keyboard-operation changes are implemented and extensively tested without mouse input.