Refactor plugin-api `addKeyboardShortcut` to point to `KeyboardShortcuts`.
* Do not add shortcuts to the default object directly.
* Create an addShortcut function in keyboard-shortcuts to add shortcuts safely and call to bindKey to be able to use opts.
* Refactor controllers/bookmark.js to use new addShortcut func and emove unnecessary addBindings.
* No longer export keyboard shortcut bindings, rename to DEFAULT_BINDINGS and remove export, these do not need to be accessed by anything else.
