rules to set `occurrences` property

MuttumAlphabet is created independently from the word, so we need to be
able to update `occcurences` letter property after their construction.

When we will create MuttumEngine, the MuttumLanguage object will be a
property of the MuttumEngine. So, the caller of the MuttumEngine will be
responsible to initiate the MuttumLanguage as desired by the user
(dictionary file, word length constraints).

This will allow too to avoid to rebuild MuttumLanguage between each
party, because there's no need to parse again the entire dictionary file
if it's not changed.

Param spec property description is considered deprecated for GTK related
libraries.

the "property warning" test can't reach the switch default case,
certainly because GObject already raise the warning before our code can
do it.

All current properties have static name, nickname and blurb, so we can
apply the documentation recommendation to improve internal performance
of GObject.

MuttumLanguage contains all tools to create dictionary and analyze
graphemes according to a given locale setting.