Add keybind customization

[mitchcapper]

Initial keybind customization work, note this is only configurable in the settings json but you can do things like:

"KeyBindOverrides": {
  	"NewCSXScript": "Control+U",
  	"NewCSDocument": "Control+Shift+U"

  }

See the enum for all key binds.
Note: While I replaced all the UI Keybind calls in the two main docs for windows this is largely a POC commit. I also show two examples of how this could be done. One is where the view directly accesses a model data type, it is a bit cleaner IMOP but not exactly binding to the VM:

• ToolTip="{Binding Converter={StaticResource KeyBindToDescriptionConverter}, ConverterParameter={x:Static ui:KEY_BIND.RunScript}}"
• The other example is more MVVM and what i did for most but requires adding another field for each key bind to the viewmodel:
  Key="{Binding SaveGesture, Mode=OneTime, Converter={StaticResource KeySequenceToGestureConverter}}" (with SaveGesture added to the VM)

You will also note some stupidity where I have to bind both Key and Modifiers separately, this is unavoidable I believe (as Gesture is not a dependency property).

Assuming you want this let me know which style you prefer ill update all the ones I can find in the code and update the PR.

Note: AvalonEdit does not support key bind changes easily either. We can add binds but not really remove (I believe). I can make a PR on it though to add that support if we want to proceed to allow rebinding those too.

Closes #272