Add Replace in... to context menus

Add "Replace in..." to project tree and working set context menus.

https://trello.com/c/3cKG28Fn/1309-replace-in-from-file-tree

cc @njx 

@njx Would it be an issue to change `expectInMemoryFiles()` to not expect any other files to be open in the working set?  Might as well give all tests the benefit of that extra check while we're at it...