v2.5.1

Added

• Linux: a "+" beside Categories in the Tasks sidebar adds a category (it shows even for a list
  without any), and a right-click or long press on a category offers Edit… to rename it on all
  the list's tasks, as one undo step.

Fixed

• Linux: dropping a task in a long list with manual sorting no longer freezes the window for
  seconds; the page is rebuilt once after a burst of changes instead of once per renumbered task.