gh-89520: Load extension settings and keybindings from user config (GH-28713)

Extension keybindings defined in ~/.idlerc/config-extensions.cfg were silently ignored because GetExtensionKeys, __GetRawExtensionKeys, and GetExtensionBindings only checked default config. Fix these to check user config as well, and update the extensions config dialog to handle user-only extensions correctly.

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Co-authored-by: blurb-it[bot] <43283697+blurb-it[bot]@users.noreply.github.com>
Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>
Co-authored-by: Gregory P. Smith <68491+gpshead@users.noreply.github.com>
Co-authored-by: Gregory P. Smith <greg@krypto.org>