[Refactor] Major refactoring changes

- Refactored mod_CUsMgr: moved some of its content into mod_CUsMgr, mod_main, mod_utils, mod_selectFocusables and mod_domEvents
- Renamed `miscGlobalSettings` to `miscSettings`
- Renamed `mod_events` to `mod_pubSub` to prevent confusion with the newly created `mod_domEvents`
- Comment out the feature to "search external sites" since it should be a separate (tiny) extension
- In `_setupActionShortcuts` and `_setupMUsShortcuts` add a hasOwnProperty() check while iterating