Improve "Attach to Unity Process" dialog

[citizenmatt]

This PR improves the UX of the Attach to Unity Process dialog, and fixes a few bugs in player discovery

• Editors and players are now grouped by project, when available (grouped by "Unknown project" when not)
• Editor helper processes are shown as child nodes in a tree view
• Within groups, processes are listed in order of importance - editors, then editor helpers, local/standalone players, remote players
• Local processes, such as editors and editor helpers are refreshed once a second, so new processes are added/removed while the dialog is open
• Local process discovery now only checks if a process is called "Unity", rather than trying to match heuristics. This mimics what Unity do in their own debugger plugins for other IDEs
• Player discovery is rewritten and improved:
  • Previously, polling would read one broadcast message from a socket and sleep, while more messages are being sent, creating a backlog that is never properly processed. This could prevent all players being correctly listed. Now we read all available messages
  • Previously, polling was handled with blocking and timeouts. This has been rewritten to use select style semantics - only do something if the socket has data
• Unity specific run configurations are now only available in Unity projects, and are dumb aware