Latvian: finally handles PassV2 clauses in a more or less adequate way

Extends the previously introduced support for specifying the verb valence (verb-dependent subject and object cases), involving functions up to PredVP and RelVP.

In Latvian, the passive voice is not used if the agent (subject) is known; to preserve the information structure (i.e. the word order), a clause like 'A is <done> by B' is linearized in the active voice ('A <does> B') where A has the object case (e.g. Acc), and B - the subject case (e.g. Nom). Thus, the verb valence patterns are swapped on-the-fly.

This is still a rather quick & dirty implementation: parameters and linearization types have to be optimized (VerbLav), the use of PassV2 in AdvVP is problematic as it doesn't apply VPSlashPrep / ComplSlash, etc.

lib/src/latvian/ExtraLavAbs.gf

@@ -8,7 +8,7 @@ abstract ExtraLavAbs = Extra ** open ResLav, Prelude in {
     kopsh_Prep : Prep ;
     liidz_Prep : Prep ;
     pa_Prep : Prep ;
-    par_Prep : Prep ;
+    --par_Prep : Prep ;
     paar_Prep : Prep ;
     pie_Prep : Prep ;
     pret_Prep : Prep ;