Various improvements:
- Switched to ReadOnlyDictionary in Visualization to make it clear
that the parameter dictionary should not be modified.
- Added a warning to the Visualization Set editor that appears when
there are no plugins that implement a visualizer.
- Make sure an item is selected in the set editor after edit/remove.
- Replaced the checkerboard background with one that's a little bit
more grey, so it's more distinct from white pixel data.
- Added a new Apple II hi-res color converter whose output more
closely matches KEGS and AppleWin RGB.
- Added VisHiRes.cs to some Apple II system definitions.
- Added some test bitmaps for Apple II hi-res to the test directory.
(These are not part of an automated test.)
Thumbnails are now visible in the main list and in the visualization
set editor. They're generated on first need, and regenerated when
the set of plugins changes.
Added a checkerboard background for the visualization editor bitmap
preview. (It looks all official now.)
