Various changes:
- Generally treat visualization sets like long comments and notes
when it comes to defining data region boundaries. (We were doing
this for selections; now we're also doing it for format-as-word
and in the data analyzer when scanning for strings/fill.)
- Clear the visualization cache when the address map is altered.
This is necessary for visualizers that dereference addresses.
- Read the Apple II screen image from a series of addresses rather
than a series of offsets. This allows it to work when the image
is contiguous in memory but split into chunks in the file.
- Put 1 pixel of padding around the images in the main code list,
so they don't blend into the background.
- Remember the last visualizer used, so we can re-use it the next
time the user selects "new".
- Move min-size hack from Loaded to ContentRendered, as it apparently
spoils CenterOwner placement.
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