Get rid of Preferences dialog

I think we can completely get rid of the preferences dialog in Files. Most of the options available should exist as either system-wide options or not at all.

Behavior:

Single click to Open - Should be a system-wide preference if we decide to allow this preference

Auto-selection Speed - Should have a sane default, which in my experience is 0 since this doesn't work very well. Maybe better as a dconf key for more experienced users.

Interface:

Sidebar Icon size: should be a system-wide pref if we decide to allow this

Date format: should also be system wide.

Extensions:

These should be enabled if installed and disabled if not installed. I think it would be better to move extension management into AppCenter. This moves a complex feature (that when implemented fully needs to involved finding and installing extensions) into a place where it makes more sense: the package manager.

And that's all the prefs, poof gone.