#26318 Added a global engine busy popup window

Adds *internal* functionality to pop a busy UI to indicate that long
running activity is happening. The intent is to use this for the path
cache sync scenarios which may take longer than usual, for example a
full sync. The progress only pops up on engines which have UI (QT)
capabilities - other engines will log an info message instead.

Closes #100 

(Happy 100 Core Pull requests!:balloon:)