Allow engine to define its own variant names

Non-standard variants will appear in the New Variant menu, and when
selected will lead to sending of the engine-defined name in the variant
comand to the engine. Which then should report with a setup commandto specify parent variant, board & holdings size, piece-to-char table and
iitial position.
Fix engine-defined names
Fix variant choice for second engine
Implement (inaccessible) dark squares
Make XBoard xpm-free

The icons in the Engine-Output window were the only pixmaps still in use.
They are now replaced by bitmaps, like the checkmark already was.
BUILD: make paths modifiable (tiny change)

    This can help with setups where games are installed in their
    own location, but system data files like desktopfiles
    should still go to /usr/share.

        https://savannah.gnu.org/bugs/index.php?39969
BUILD: fix configure switches (tiny change)
BUILD: make Xaw frontend default (tiny change)
BUILD: fix withXaw conditional (tiny change)

Listing both one after another leads to withXaw being false
if you pass "--with-Xaw3d --without-Xaw".
Updated German translation
Updated Ukrainian translations
Added Dutch translation
Translation: fixed some inconsistencies reported by Benno Schulenberg
fixed some whitespace issues in configure.ac
configure.ac: don't set xaw if we choose gtk
expose the configure options to xboard
output configure options when looking at --version
fixed some more translation strings
more translations fixes: use uppercase for variant names
updated Dutch translation