Added support for multigraph in click_on_letter

The impact for translators is that the alphabet now is:
a/b/c/d/e/f/g/h/i/j/k/l/m/n/o/p/q/r/s/t/u/v/w/x/y/z

instead of:
abcdefghijklmnopqrstuvwxyz

and in te destop file for le levels, letters get separated by blank
spaces like this:
Questions=a e i o u y
Answers=a e i o u y c s

Added a translator comment to the relevant string.

This meant a complete rewrite of the data structures, both for the
string in the .po catalog and the desktop file. Using gchar** instead
of gchar* for the alphabet, and for the questions/anwsers use GSList
for dealing with the dynamic length without running into memory
allocation troubles.

Improved shuffling algo where it doesn't need any reordering just to
get at some data structure.

Switched the question and answer columns in the config, because the
order was confusing (having the answers before the questions).