Disentangle datestring and textdate features

Qt's public date/time APIs have some deprecated parts dependent on feature textdate and some other parts dependent on feature datestring. One of datestring's string format options is Qt::TextDate; some (but not all) implementations of datestring APIs have their Qt::TextDate parts #if-ed on the textdate feature; but the qnamespace.h declaration of Qt::TextDate isn't conditioned on textdate. Indeed qnamespace.qdoc describes Qt::TextDate as "The default Qt format" and various datestring functions take it as their default value for a Qt::DateFormat variable. Because some implementations depend on textdate, datestring is declared in corelib/configure.json to depend on textdate.

The two features are both public. They describe themselves as:

• textdate - Supports month and day names in dates.
• datestring - Provides conversion between dates and strings.

I propose that, when textdate is disabled, datestring's APIs treat Qt::TextDate as Qt::ISODate; with this change, datestring is decoupled from (i.e. can be enabled without) textdate. This can pave the way for making textdate entirely optional. It only changes API or ABI compatibility when datestring is enabled without textdate, which was not possible before this change.

Open questions (assign to me once answered):

• Any objections to this plan ?
• Which branch ?