Add -installTheme option

Add -installTheme option

A new option of type ArgInstall is added, which will add its argument
to the -themeNames list (under the usual restriction it is not already
in there, and that it was not seen before). By appending such a (time-
stamped) option to the master settings file with -addMasterOption,
newly installed themes can be made to trickle their way into the user's
private settings file. All the theme package has to is call XBoard with
the applicable -addMasterOption "-installTheme ...." arguments in its
install script, and copy any associated files to XBoard's datadir.