getRecurrenceString() function needs to be rewritten

The getRecurrenceString() suffers from different issues:

- It doesn't have translatable strings for recurring events happening weekly on a given day
- It uses chained ? : statements that make the code difficult to read (where if/else statements could be used)
- It redefines Qt enums as numerical indexes to be used for different purposes, indexing global arrays outside of the function. That makes the code extremely difficult to follow, and very prone to errors.

I'd suggest reviewing and rewriting it addressing those issues.