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.)
