Replace occurrences of "PyFace" with "Pyface"

There are many references to "PyFace" within this codebase, but the name of the package should be "Pyface" rather than "PyFace".

Almost all of these references are in documentation or comments. The one exception I can see is in traitsui.menu, where there's a `from pyface.action.api import Action as PyFaceAction`. That's probably safe to change - it seems unlikely that anyone is directly importing `PyFaceAction` from `traitsui.menu`.