gbui 0.3.1

A portability fix, and the two jobs that caught it. The library's own sources
are the same ones 0.3 shipped; what would not build is the test suite on a Mac
and the sample that links against an installed prefix.

Fixed

• Modifiers had a reader for the platform's shortcut key and no writer.
  command() answers "Ctrl, or Cmd on a Mac", so everything that had to make
  such an event wrote the #ifdef again instead — two test harnesses so far,
  and the second spelt the field meta, which is not a field. Only one branch
  of a copy like that ever compiles, so the machine you are on cannot tell you:
  it built here and on both Linux jobs and failed on macOS. withCommand()
  holds down whichever key the reader reads, and both harnesses now call it.
• components.hpp stopped exporting text and button and nothing noticed.
  Splitting "controls" into elements and components moved them to
  elements.hpp, which is what the guide has told callers to include ever
  since — but the consumer sample was still on the old umbrella. No in-tree
  build catches that, because everything in the tree includes the specific
  header it wants; the consumer is the only thing that asks the installed
  prefix for an umbrella and finds out what is in it. It now includes all four,
  which is the whole point of it.