@next +robustness single-app model with remote-open, to prevent overwriting of the list of opened files by two separate copies of the app
@tickler +feature support todo.txt / done.txt (moving done tasks to a done.txt file, like Thunderbird's GTD)
@tickler +ui dynamic toolbar -- show which buttons on the toolbar based on which pane is focused.
@tickler +ui toolbar actions: add / remove tasks when focused on task view.  open question: should add task keyboard shortcut work when task view is not in focus?
@tickler +ui improve how things look when they are highlighted, and make which pane is focused more obvious
@next +ui mark selected task(s) as complete with keyboard (F12 to toggle, figure out if a checkbox is also worth trying)
* more toolbar actions (activatable via hotkeys) that apply to selection: add remove tags / lists with a popover menu / possibly typeahead find
* context menus to do all the above
* custom filters with filter language (implement task objects)
* custom sorting
* grouping
* fast data entry (possibly with autocomplete tags and lists)
  * this may require moving away from the simple text box we use for entry right now
  * this is gonna look awesome, we will overlay a widget that will display +tags and @cats and other metadata with nice boxes
* automatic creation date addition
* undo capability
+robustness @next ensure that trying to load invalid non-utf-8 files actually displays an error instead of silently failing and leaving half-initialized stuff
x search filter box
x exit app toolbar button and key combo
x multi-window: allow new window to be created with key combo and toolbar button
x multi-window: open a file to create an entirely new window
x configure where todo.txt / done.txt are saved
