Clean up creation and use of KeyStroke class

[edreamleo]

This is an experimental project. It is made feasible because of git branches and Leo's cff command.

You would think that most of Leo's code could easily use strokes (instances of the KeyStroke class). But that's not true. There are two ways to attempt simplifications:

1. Discover all places where strokes are created and used.
2. Attempt to create new distinctions that are latent in the KeyStroke class.

As part of this simplification, it seems reasonable to move the code that creates KeyStroke objects from keyboard codes should move from the LeoQtEventFilter class into the LeoQtGui class.

[edreamleo]

I am going to close this project. #423 supersedes it.