Fix several old issues with indexed images and transparent color

- If we use a RGB color in a background layer (indexed image),
  and the first palette entry (transparent color) matches that RGB color,
  we can use that entry, because this is the background (the transparent
  color isn't transparent at all in the background layer).
- If we use the same RGB color in a transparent color, we've to avoid the
  transparent color, because the user want to paint with that specific
  solid color.
- Palette::findBestfit() receives a mask_index now, so we can find what
  color best matches a specific RGB avoiding that specific mask color,
  or we can use -1 to use any color (e.g. for background layers).
- Added app::ColorTarget() to simplify and fix color_utils::color_for_layer
  and app_get_color_to_clear_layer(), so now we can use the new findBestfit
  to return the transparent color for background layers (if a RGB color
  matches the transparent color RGB values).
- Removed fixup_color_for_layer/background() functions in color_utils
- Fix NewImageFromMask() to use the mask color to clear the image
- Improve the Editor pen preview (cursor.cpp) to draw the bounds of the
  pen when it will paint with the transparent color.
Add command to switch onion skin visibility
Don't show onion skin in the mini editor