[METATICKET] Tree view enhancements

[Janek Kozicki]

• Active + mouse clicks doesn't refresh view (#2216)
  • master + patches from #1517 #2214 tickets
• hidden subdirectories
  • display whether the contained subdirectories ▶ are all hidden ones (dot-files) or not (it can be annoying if you try to unfold, and nothing unfolds, because the subdirectories are hidden)
• unfolding hidden subdirectories
  • Second press of Enter on a collapsed directory, which contains only hidden subdirectories, should unfold and show those hidden directories
• speed up detection of subdirectories
  • speed up detection whether a directory has subdirectories, by caching this information in the tree_entry struct: (mtime timestamp + yes|no)
• display symlinked directories
  • Curerntly tree view ignores symlinked directories. It's impossible to browse them.
• forget everything but this
  • Improve F3 "Forget-this" option by adding another "Forget-everything-except-this"
• folding/collapsing makes tree_navigation_flag less useful (#2214)
• unfold recursively
  • Make it possible to unfold current dir recursively: with all subchildren. Except for symlinked directories, because that could lead to inifite directory loop (or detect such loops).
• add mouse scroll support
• to consider - making clickable hot spots in tree view
  • Currently unfolding happens only when double clicking on directory in tree view
  • A mouse hot spot on ▶ would allow unfolding by single click on ▶ and collapsing by single click on ▼
  • Perhaps another hotspot for "forget everything but this", and another hotspot for unfolding with hidden directories?
• user manual update
  • Don't forget to update the user manual, after doing all that work on tree view