stbt.Keyboard: Add `modes` attribute
stbt.Keyboard: Add `modes` attribute

I found it useful when writing unit tests for stbt.Keyboard, and it'll
be useful if we want to add more error-checking when you're defining the
keyboard model.

stb-tester/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester