System Status Menu: implement some kind of quick settings

Quick settings or actions are found across many other platforms*, they expose commonly used settings to people so they can toggle them at their whim.

This is something we should implement in the Shell as well since having quick settings would a) help you get to settings faster since they'd be front and center in the status menu (opposed to in a submenu or off in the Settings app) and b) show the state of common settings right there in the menu.

Presently we can toggle settings in the menu presently but the few we do expose are hidden under a submenu as "turn off" which is not exactly quick nor very suitable to touchscreen input. Replacing these with a button that toggles the setting as well as more prominently indicates the state (this is a status menu after all) with colour and an icon would be an improvement.

Conceptual Solution

Here's a concept for an overhauled status menu with quick settings. (A more detailed breakdown of this concept with explanations for the changes is here: https://gitlab.gnome.org/Teams/Design/os-mockups/raw/master/system-status/status-menu+quick-settings.png)

The gist here is that quick settings would be shown in a grid of buttons, that with both colour and icons show the state of various toggleable settings. Some items that were previously menus indicate that they still have extended options in their labels and clicking those opens those.

Like on other platforms, this grid could also be extensible and customizable since settings relevance will vary person to person but some common ones should always be present.

*Relevant Examples

Windows 10 Quick Actions

The Quick settings on Android/Chrome OS

Control Center on iOS on iPad/iPhone