Bug fix for restarting global plugins

- fixed a bug where some global plugins would not recover sufficiently when
    dynamically restarted
Updates to font handling

- updates to font handling
  - user can now set a fixedFont and a sansFont
- thumbnail handling changed to add a predictive preload of files
  - when clicking a thumbnail to load it, it can load the file before and after
    in the list.  This makes for faster switching between images when the
    user is moving through them sequentially.
  - this is controlled by a preloadImages preference in the Thumbs preferences.
      Default is off.
- default keyboard mapping changed:
   - "<" and ">" now interactively set low and high cut levels, respectively
   - "." is bound to a two-axis interactive cut levels