Bring key mappings up to date with tcell

[TripleDogDare]

This adds missing string mappings for the named key codes (Should address parts of #41). (Except for KeyCtrlLeftSq which is Escape). Unfortunately I don't see anything for additional media keys and I'm pretty sure those would have to be surfaced by tcell as currently implemented.

[marcusolsson]

I tried running xdotool key XF86AudioPlay and like you say, it doesn't seem like media keys are supported by tcell. The idea from the start has been to rely on a third-party library for the actual communication with the terminal. In fact, originally tui-go was termbox-go for this, but I later felt tcell supported more of the things I had imagined for tui.

If media keys turns out to be a highly requested feature, then I think that it should preferably be implemented in tcell first

Either way, thanks for your contribution! Much appreciated!