Allow custom xkb keyboards to be displayed

Advanced users can define custom Xkb keyboard layouts for otherwise
unsupported languages/scripts.  Without this change, only the standard
system keyboards could be seen in our programs even though the system
could choose them through its standard setup software and input language
applets.
Fix keyboard layout loading for custom keyboards
More fixes for listing custom xkb keyboards on Linux
Merge pull request #53 from sillsdev/FixForCustomXkbKeyboards

Allow custom xkb keyboards to be displayed