General improvement and bug fixes.

Allow the choice of only one set of QT limits at a time.  Although the previous system could allow choosing criteria for both long and short QT intervals at the same time, some combinations of criteria made no real sense, so this version doesn't allow multiple criteria at the same time.
Fix some layout problems.
Update icon.

Fix bug preventing opening links to source references

Add navigation buttons to Help

iOS and macOS versions combined into same app bundle

Improved help screen
New icon

Dark mode support

Add dialog confirmation of graphs saved to photos
User interface improvements

Add QTcRBK (Rabkin et al., 1. Rabkin SW, Szefer E, Thompson DJS. A New QT Interval Correction Formulae to Adjust for Increases in Heart Rate. JACC: Clinical Electrophysiology. 2017;3(7):756-766. doi:10.1016/j.jacep.2016.12.005)

Minor changes to Help text

Minor bug fixes