Add floating mode for MacOS

[yarray]

Add options to make the window float over any (fullscreen) apps on MacOS:

1. Add a "floating" config.
2. Add a "Turn On Floating Mode" button in menu -> preferences. When click:
3. Set "floating" to true, reload the app. The dock icon and the menu will be hidden, while the menu bar icon will appear. The window can overlay any app now.
4. Add a "Turn Off Floating Mode" button in menu bar icon's menu. when click:
5. Set "floating" to false. Reload the app. The app will function as normal.

The mode will be useful when a user wants to quickly view a doc. A floating mode will minimize distraction since it does not force jumping to another space.

This pull request will not affect the app's behavior unless a user clicks "Turn On Floating Mode".

[egoist]

Maybe adding a "pin" icon (show it when hovering window header) on the top right corner of the window, and clicking on it to toggle "float" mode is more convenient 🤔

[yarray]

Maybe adding a "pin" icon (show it when hovering window header) on the top right corner of the window, and clicking on it to toggle "float" mode is more convenient 🤔

Sounds good. I may have a try. Not sure whether not relaunching will cause problems though.

[yarray]

@egoist I've add a pin button on the title bar. It could be more visually obvious but I think the convenience has been achieved.

Also I keep the relaunching behavior since I cannot reliably switch floating mode without relaunching.