Save open plug state

[cassidyjames]

After a discussion with @danrabbit, we concluded that System Settings was unique in that closing the window is not like minimizing, where the state is saved. Even if users may have a more linear flow with system settings, being able to open it and tweak a setting, close it to try the setting out, and re-open to further modify would be more consistent with how other apps work on elementary OS.

The main question I have is: how should this be implemented technically? My first instinct is to save the opened plug, but some plugs have multiple tabs and ideally we'd save that state as well. Would somehow saving the settings URI be possible, then we could use that when opening System Settings itself (without passing in a settings URI) to navigate back to that plug/subpage?

Ideally the following will all work:

• Opening System Settings from the .desktop will restore the plug and tab that was previously open

• Opening a specific plug/tab from Applications Menu search will still work and override the saved state

• Opening a specific settings://foo/bar URL will similarly still navigate to the correct plug/tab, overriding the saved state

• Opening settings:// without any further path will open the main screen instead of the saved state