Cleanup: use plural names for Main lists

Convention was not to but after discussion on 918941483f we agree its
best to change the convention.

Names now mostly follow RNA.

Some exceptions:

- Use 'nodetrees' instead of 'nodegroups'
  since the struct is called NodeTree.
- Use 'gpencils' instead of 'grease_pencil'
  since 'gpencil' is a common abbreviation in the C code.

Other exceptions:

- Leave 'wm' as it's a list of one.
- Leave 'ipo' as is for versioning.

source/blender/python/generic/bpy_internal_import.c

@@ -186,7 +186,7 @@ PyObject *bpy_text_import_name(const char *name, int *found)
 	memcpy(txtname, name, namelen);
 	memcpy(&txtname[namelen], ".py", 4);
 
-	text = BLI_findstring(&maggie->text, txtname, offsetof(ID, name) + 2);
+	text = BLI_findstring(&maggie->texts, txtname, offsetof(ID, name) + 2);
 
 	if (text) {
 		*found = 1;
@@ -196,7 +196,7 @@ PyObject *bpy_text_import_name(const char *name, int *found)
 	/* If we still haven't found the module try additional modules form bpy_import_main_list */
 	maggie = bpy_import_main_list.first;
 	while (maggie && !text) {
-		text = BLI_findstring(&maggie->text, txtname, offsetof(ID, name) + 2);
+		text = BLI_findstring(&maggie->texts, txtname, offsetof(ID, name) + 2);
 		maggie = maggie->next;
 	}
 
@@ -245,7 +245,7 @@ PyObject *bpy_text_reimport(PyObject *module, int *found)
 	}
 
 	/* look up the text object */
-	text = BLI_findstring(&maggie->text, BLI_path_basename(filepath), offsetof(ID, name) + 2);
+	text = BLI_findstring(&maggie->texts, BLI_path_basename(filepath), offsetof(ID, name) + 2);
 
 	/* uh-oh.... didn't find it */
 	if (!text)