Folder and file with the same (base)name gets ordered by creation time for `a-z` and `true a-z` orders

[SebastianMC]

Following this suggestion from @cmasfo in discussion thread #122, remove the randomness.

Potential solution options include:

• assume some reasonable behavior, like if none of the sorting levels (5 supported now) puts two items in order, let file go before a folder (if one item is a file and the other is a folder)
  • this option is preferred for simplicity
• introduce new explicit sorting order which allows specifying folders or files to go first
  • tempting, yet because it would consume one level of sorting specifications, other options should also be exposed

[SebastianMC]

Done.

• Added explicit syntax to specify files-first or folders-first orders
• the last-resort default fallback sorting method (which was alphabetical) is extended to give preference to files. In other words, if all the specified sorting levels don't oder two items, the file goes first (if the other item is a folder)