Map letters by key codes

This switches the processing of letter keys to use key codes instead of
textual input.  The process we are simulating here is the core command
routing of Visual Studio.  This occurs during PreTranslateMessage and
looks at virtual key codes.  Looking at the WPF key entries directly
here gives us the closest behavior.
Only fallback in vim buffer if VsVim disabled

My previous change was simply broken for vim buffers.  It was always
routing keyboard input through the fallback processor.  This fixes this
by only routing to the fallback processor on a vim window if VsVim is
currently disabled
Merge pull request #1 from jaredpar/fixes-925

Map letters by key codes