todo.txt: view mode - Fancy grouping, sort, search etc

It is possible in markor to sort the entries in edit mode. This is not very convenient, as it may be desirable to view some ordering for the entries but keep the natural ordering in the file. This is convenient so as older entries are at the end of the file. One possible advantage is for using a version control system for the file, to compare with older files etc.

My proposal is to add a button to sort entries in view mode. Files does not change, only visualization. It should be possible to sort by project, date, priority, etc. (decreasing and increasing)

Also, it would be nice to group by projects.

No files or tests are named. Start by locating Markor's todo.txt view mode and its existing edit-mode sorting, then define the view-only behavior requested: sorting by project, date, and priority in both directions, plus project grouping, without changing the file contents.

Written by the indexing model from the issue text.