Adding new target range indicator to calendar event title

[gonkowonko]

This PR is to add a new visual indicator to the calendar event title, which is a emoji circle coloured in the same way as the Urgent/Not Urgent and In Range font colours.

• New boolean setting under "Calendar Events" called "Display Visual Indicator?"
• Added extension method to BgReading that returns a "BgRange" value which based on the BgReading value returns what range the Bg value falls between based on the user's objectives.
• New const to store the enum value of the BgRange.
• Added new properties to the Calendar constants which store the emoji icons.
• Extended the calendar creation event to add the appropriate icon before the title if the setting is enabled.

Sample Screenshots

[paulplant]

Pulled into a new branch to continue work...