Make Window Switcher primary modifier configurable

### Problem

I set the window switch shortcuts to "Ctrl + Alt + Tab" and "Ctrl + Shift + Alt + Tab". When switching, keeping Ctrl pressed, but letting go of Alt and Tab exits the window switcher. I would like the window switcher to stay on if Ctrl is pressed.

### Proposal

I took a look at the code, and the logic for deciding which modifier is the primary modifier is in `WindowSwitcher.vala`, in the `handle_switch_windows` method. There's a call to `binding.get_mask ()`, and the primary modifier is computed from that. In the above case, the primary modifier is computed as Alt, but I would like to configure it to be Ctrl.

https://github.com/elementary/gala/blob/5a75065ff52e1b009a02fc338345e850849a81d8/src/Widgets/WindowSwitcher/WindowSwitcher.vala#L114

### Prior Art (Optional)

The current behaviour when switching is keeping Alt pressed, but letting go of Ctrl and Tab keeps the window switcher on. I just want the roles of Alt and Ctrl swapped.