The main refactoring of ConfigDialog into multiple classes is done.  Since you kept this patch up to date with the changes, I will review it next, before making further changing to individual tab pages.

On problem with implementing some features as extensions is that associated keybindings are global to all keysets, instead of being specific to one keyset.  This meant that the defaults were necessarily the same for Windows, *nix, and OSX.  (Not necessarily bad, but inflexible.)  This means that any change changes the supposedly fixed built-in keysets and that users cannot try out different bindings in different custom keysets.  This strikes me a  something of a bug, so I am looking more favorably at moving all keybindings (other than true, 3rd-party extensions) to config-keys.def/cfg.