substantially refine the path x-platform path displaying

- File::name is now File::decodedName
- new File::displayName which properly applies separators to the name
  (on Windows src/ should be src\)

The motivation behind this change is to force us having to make a decision
if we want the low-overhead name or the high-overhead UI name.

- File::fullPath is now File::displayPath for the same reason as above
- new File::url as a convenience wrapper used by the display* functions
  to construct a full URL out of their tree so we can determine whether
  the path is local or remote (scheme file:// vs. http:// etc.)
- RadialMap::Widget::mouseMoveEvent had its tooltip adjusted to not
  resolve it against the held tree but rather the entities tree. This was
  inconsistent with hover signals emitted and caused remote paths to
  get improperly displayed like local paths in the tooltip while being
  correctly displayed as URLs in the rest of the UI