Standardize constructor method names #1203

There are currently no standardized naming conventions for constructor methods. For example, consider these methods:

QDate::from_string_enum(string: &QString, format: DateFormat) -> Option<Self>
QDateTime::from_string(string: &QString, format: DateFormat) -> Option<Self>
QTime::from_string_enum(string: &QString, format: DateFormat) -> Self

QDate::from_string_enum and QDateTime::from_string have the same signature, but different names.
QDate::from_string_enum and QTime::from_string_enum have the same name, but different signatures.

Proposal 1: Constructor methods should be the names of each parameter type, prefixed by from_. If they return Option<Self> rather than Self, the name should be suffixed by _opt, which is a common pattern in other crates. So the above would be replaced by:

QDate::from_qstring_dateformat_opt(string: &QString, format: DateFormat) -> Option<Self>
QDateTime::from_qstring_dateformat_opt(string: &QString, format: DateFormat) -> Option<Self>
QTime::from_qstring_dateformat(string: &QString, format: DateFormat) -> Self

Proposal 2: Deprecate all the constructor methods and just use From and TryFrom with tuples. So the above would be replaced by:

impl TryFrom<(&QString, DateFormat)> for QDate

impl TryFrom<(&QString, DateFormat)> for QDateTime

impl From<(&QString, DateFormat)> for QTime

Proposal 3: Do proposal 1 and the implementations from proposal 2.