cxx-qt-lib: standardize constructor method naming conventions

[jnbooth]

Per #1203, constructor methods are now named directly after their parameters, and suffixed by _opt if they return Option<Self>.

This PR also adds the AnyDateFormat type (an enum of either a DateFormat or a QString) in order to deduplicate methods for converting between date/time types and QStrings:

// Old
QDate::from_string(mystring, &QString::from("myformat"));
QDate::from_string_enum(mystring, DateFormat::ISODate);
date.format(&QString::from("myformat"));
date.format_enum(DateFormat::ISODate);

// New
QDate::from_qstring_opt(mystring, &QString::from("myformat"));
QDate::from_qstring_opt(mystring, DateFormat::ISODate);
date.to_qstring(&QString::from("myformat"));
date.to_qstring(DateFormat::ISODate);

Alternatively, we could get rid of AnyDateFormat and use date.to_qstring_qstring, QDateTime::from_qstring_dateformat_opt, etc. I think the AnyDateFormat approach is slightly cleaner, but I'll get rid of it if you prefer.

These are all breaking changes. Alternatively, the old functions could be deprecated rather than removed. I didn't see any #[deprecated] tags in the codebase, so I defaulted to removing them.

Full list of changes (copied from the changelog):

• QTime::from_string and QTime::from_string_enum now return Option<QTime> rather than QTime, in keeping with the identical functions for QDate and QDateTime.
• Combined and renamed the following functions:
  • QDate::from_string and QDate::from_string_enum → QDate::from_qstring_opt
  • QDate::format and QDate::format_enum → QDate::to_qstring
  • QDateTime::from_date_and_time_time_zone → QDateTime::from_qdate_qtime_qtimezone
  • QDateTime::from_date_and_time_time_spec → QDateTime::from_qdate_qtime_timespec
  • QDateTime::from_string and QDateTime::from_string_enum → QDateTime::from_qstring_opt
  • QDateTime::format and QDateTime::format_enum → QDateTime::to_qstring
  • QImage::from_data → QImage::from_data_opt
  • QTime::from_string and QTime::from_string_enum → QTime::from_qstring_opt
  • QTime::format and QTime::format_enum → QTime::to_qstring

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (1e80af7) to head (e903c02).

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #1220   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           73        73           
  Lines        12593     12593           
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  Hits         12593     12593           

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