took tense linearizations apart from Common/Cat to Tense, for bettter structure of lib

lib/src/abstract/Tense.gf

@@ -0,0 +1,22 @@
+--1 Common: Structures with Common Implementations.
+
+-- This module defines the abstract parameters of tense, polarity, and
+-- anteriority, which are used in [``Phrase`` Phrase.html] to generate different
+-- forms of sentences. Together they give 4 x 2 x 2 = 16 sentence forms.
+
+-- These tenses are defined for all languages in the library. More tenses
+-- can be defined in the language extensions, e.g. the "passé simple" of
+-- Romance languages in [``ExtraRomance`` ../romance/ExtraRomance.gf].
+
+abstract Tense = Common ** {
+
+  fun
+    TTAnt : Tense -> Ant -> Temp ;
+
+    PPos, PNeg : Pol ;           -- I sleep/don't sleep
+
+    TPres  : Tense ;                
+    ASimul : Ant ;
+    TPast, TFut, TCond : Tense ; -- I slept/will sleep/would sleep --# notpresent
+    AAnter : Ant ;               -- I have slept                   --# notpresent
+}