feat(developer): fixes to kmc-package for keyboard support

kmc-package had a number of gaps and errors. This fixes a bunch of
problems:

* Wrong case on XML elements
* Wrong metadata in system element
* Missing kmp.json in Files array (needs test in consumer apps later)
* Missing displayFont, oskFont in keyboard elements
* Including copyLocation when == 0
* Including an empty startMenu element
* Incorrect assumptions about location of source files in source/ folder

Note that support for followKeyboardVersion will come later; for now, it
uses the possibly-stale value from the Keyboard element in the .kps
file.