More small improvements

Version 1.53.0 contains a bunch of small improvements. The main changes:

• An improvement to the Spotlight feature. If there are any squares where the spotlight value is the only candidate value, these are shown highlighted as single cases, even if there are other squares in the same row or column which also have the value as a candidate.
• The option to show inverted combinations now takes effect whenever a sum value is tapped twice, even if two blocks are not present. Previously, the combinations were only shown inverted if there were two blocks, when double tap also inverts the selection.
• Improvements to the way the difficulty of newly-generated puzzles is estimated for cases involving expert complexity.
• Improvements to the way the built-in help chooses where to show a help hint for puzzles generated in expert mode.
• Further improvements to help explanations.

​This version also includes fixes for the following issues:

• A fix to prevent false alarms for an “incorrect sum” when candidate values are being set on a single square. Previously, other rows and columns that were impacted by the selection might signal an error after the first candidate value had been set, only to be cleared again when more candidate values were added to the selected square.
• A fix for when values are applied to a group of squares that have been selected together. If the first value to be tapped was not already a candidate for any of the selected squares, the squares were being cleared instead of being updated correctly.
• A fix for when a square with a single candidate value is tapped and then the value is confirmed by tapping again. Previously, this was causing the value to be eliminated instead of being confirmed.
• A fix for an isolated crash that was reported as happening while updating a selection with multiple squares.
  
• Fixes for a few missing symbols (icons) with iOS 14.