TODO

TODO:
  - refactor the Find UI and logic from DotWindow into a FindBar
  - make the highlight_animation non-global to properly support multiple simultaneous
    instances of DotWindow in the same process
  - clean up the "multiline URL" patch and integrate it properly
  - bug: why doesn't the close button always show up when xdot is maximized
    under the Unity shell?
    (take partial screenshot with the windows key, then it bugs out)

DONE:

  x- traverse by clicking: x(t) should follow tanh-like curve, not linear
  x- use the system highlight color
  x- keyboard shortcuts for 1:1 ("1") and fit to screen ("F")
  x- tooltips for toolbar buttons, including action name and shortcut
  x- the open dialog should remember the last used directory within the same session
  x- if input graph is empty, indicate somehow that a blank file was loaded
    x- create one node saying "[No input]" and render that?
  x - clean corner cases
    - error when opening file: if no graph loaded yet, should display "[No graph loaded]"
  x - when hovering over a node, highlight all linked nodes
    (better for exploring a large graph)
  x- find by text in label (incremental search)
    - text field with the text "Search" in gray until clicked,
      then clear it and wait for user input.
    - buttons:
      - focus next (if multiple matches)
        - takes the "next" match (according to some ordering)
          and "clicks on that node".
        - if unique match, focus it
      - focus previous (if multiple matches)
        - same, with previous match.
    - make incremental an option? (may be slow)
  x- animated highlighting
  x- animated zooming
  x- when mouse cursor enters Find field, re-highlight? (don't require clicking)
  x- About screen
  x- online help: add a tutorial graph explaining hard-to-discover features
    that can be opened with a help button (all required information for the help
    is already written down in the README)
  x- make layout filter change-able at runtime
  x- keep graph coordinates and zoom level on reload
    - this is mostly useful, but can be confusing if the layout of the graph
      has changed significantly.
  x- application icon!
   - bug: why doesn't the icon show up in the launcher under Unity shell?
    - answer: the icon must be available via the current icon theme;
      under Unity, window_set_default_icon() is not enough. See
      http://askubuntu.com/questions/85601/python-application-has-no-icon-in-unity-launcher
    - cannot be fixed in code; must be taken into account when packaging.