Make strong and smooth stem width handling selectable.

Make strong and smooth stem width handling selectable.

This large change does the following:

  - Introduce command line option `--strong-stem-width=STRING' and implement
    the corresponding GUI; the options for the TTFautohint function are
    called `gray-strong-stem-width', `gdi-cleartype-strong-stem-width', and
    `dw-cleartype-strong-stem-width'.

  - Print all non-zero options into the `name' table (and not only the
    non-default values).

  - In the Qt code, use QGridLayout for everything to have finer control of
    stretching in case the GUI window gets enlarged.

  - In the GUI, remove some shortcuts which are easily reachable with an
    additional press of the tab key.