Require Enchant 2.8.19+ and drop win8 provider

Require Enchant 2.8.19 or newer so Windows can use upstream winspell.
Remove the in-tree libenchant_win8 provider project and sources.
Update runtime library probing to prefer Enchant 2 library names.

src/fe-gtk/sexy-spell-entry.c

@@ -198,14 +198,16 @@ initialize_enchant (void)
     gsize i;
     const char * const libnames[] = {
 #ifdef G_OS_WIN32
+        "libenchant-2.dll",
+        "libenchant-2-2.dll",
         "libenchant.dll",
 #endif
 #ifdef G_OS_UNIX
-        "libenchant.so.1",
         "libenchant.so.2",
         "libenchant-2.so.2",
 #endif
 #ifdef __APPLE__
+        "libenchant-2.dylib",
         "libenchant.dylib",
 #endif
     };