Visualizer improvements

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.

SourceGen/MainController.cs

@@ -2344,7 +2344,7 @@ namespace SourceGen {
                     Anattrib nextAttr = mProject.GetAnattrib(nextOffset);
                     // This must match what GroupedOffsetSetFromSelected() does.
                     if (!mProject.UserLabels.ContainsKey(nextOffset) &&
-                            !mProject.HasCommentOrNote(nextOffset) &&
+                            !mProject.HasCommentNoteOrVis(nextOffset) &&
                             thisAttr.Address == nextAttr.Address - 1) {
                         // Good to go.
                         Debug.WriteLine("Grabbing second byte from +" + nextOffset.ToString("x6"));
@@ -3329,11 +3329,8 @@ namespace SourceGen {
                     // for the uncategorized data analyzer, but very annoying if you want to
                     // slap a 16-bit numeric format on all entries in a table.
                     groupNum++;
-                } else if (mProject.HasCommentOrNote(offset)) {
-                    // Don't carry across a long comment or note.
-                    groupNum++;
-                } else if (mProject.VisualizationSets.ContainsKey(offset)) {
-                    // Don't carry across a visualization.
+                } else if (mProject.HasCommentNoteOrVis(offset)) {
+                    // Don't carry across a long comment, note, or visualization.
                     groupNum++;
                 }