When screen sharing, add option for [Always Allow]

### Problem

Using remote access programs to access the secure desktop, there is an option missing to always allow a connection or program. This prevents unattended access to the GUI.

Ref: https://www.reddit.com/r/elementaryos/comments/1opaafh/how_to_do_unattended_remote_access_now_with/

### Proposal

Add a checkbox to the screen select dialog to remember the decision, instead of having to interactively ask every time

### Prior Art (Optional)