Transition to CGEvent.tapCreate for hotkey bindings (#1012)

[NMAC427]

Replace the Carbon based HotKey library with CGEvent.tapCreate as layed out in #1012.
As a consequence, we get direct access to all key-down events, and can choose to handle them based on the current modifier state and key code. Therefore, in addition to #1012, this change also addresses all of the following issues / feature requests:

• Feature request: make it possible to distinguish left and right modifiers (left cmd, right cmd, left alt, right alt) #28
• Shortcuts using `fn` don't work properly #1011
• Supporting multiple input sources #1472

The primary user facing changes are as follows:

• Added new modifiers lcmd, rcmd, lalt, ralt, lctrl, rctrl, lshift, rshift, and fn.
• Removed support for key-mapping in the config.
• Keyboard shortcuts now get interpreted based on the currently selected keyboard layout, instead of assuming a fixed QWERTY layout. This means, for example, that on a German layout alt-ä can be used as a valid binding.