bpo-36390: IDLE: Combine region formatting methods. (GH-12481)

Rename paragraph.py to format.py and add region formatting methods
from editor.py.  Add tests for the latter.
diff --git a/Lib/idlelib/mainmenu.py b/Lib/idlelib/mainmenu.py
index 1b8dc47..b0c85cf 100644
--- a/Lib/idlelib/mainmenu.py
+++ b/Lib/idlelib/mainmenu.py
@@ -60,6 +60,7 @@
    ]),
 
  ('format', [
+   ('F_ormat Paragraph', '<<format-paragraph>>'),
    ('_Indent Region', '<<indent-region>>'),
    ('_Dedent Region', '<<dedent-region>>'),
    ('Comment _Out Region', '<<comment-region>>'),
@@ -68,7 +69,6 @@
    ('Untabify Region', '<<untabify-region>>'),
    ('Toggle Tabs', '<<toggle-tabs>>'),
    ('New Indent Width', '<<change-indentwidth>>'),
-   ('F_ormat Paragraph', '<<format-paragraph>>'),
    ('S_trip Trailing Whitespace', '<<do-rstrip>>'),
    ]),