Convert IDLE's built-in 'extensions' to regular features.

About 10 IDLE features were implemented as supposedly optional
extensions.  Their different behavior could be confusing or worse for
users and not good for maintenance.  Hence the conversion.

The main difference for users is that user configurable key bindings
for builtin features are now handled uniformly.  Now, editing a binding
in a keyset only affects its value in the keyset.  All bindings are
defined together in the system-specific default keysets in config-
extensions.def.  All custom keysets are saved as a whole in config-
extension.cfg.  All take effect as soon as one clicks Apply or Ok.

The affected events are '<<force-open-completions>>', '<<expand-word>>',
'<<force-open-calltip>>', '<<flash-paren>>', '<<format-paragraph>>',
'<<run-module>>', '<<check-module>>', and '<<zoom-height>>'.  Any
(global) customizations made before 3.6.3 will not affect their keyset-
specific customization after 3.6.3. and vice versa.

Inital patch by Charles Wohlganger.
