Module for designing keyboard layouts

Changes for 0.79

• Clarify a bit in variable-VK-by-modifiers section of the docs. Revert the advice to avoid including the bitmap for lCtrl into one for rAlt. Clarify via an example. Revert: remove the stripping of lC-flags from rA in MODIFIERS (but keep the useful part of the comment). A few minor POD errors. Document "literal" in the ComposeKey. Ctrl-2 was not mentioned in the docs.
• examples/dumper-msg.c Now almost works. (Only the detection of lCtrl when rAlt is down is processed in a brain-damaged way.)