fix QDateTime performance on Qt6

[alexagr]

Switching from Qt 5.15.2 to Qt6.5.1 on Windows platform introduces HUGE performance regression in parsing of date fields when opening the XLSX files. On my laptop (i7-10510U, 32 GB RAM, Windows 10 Pro) opening of 2 MB file takes roughly 3 sec on Qt 5.15.2 and more than 3 minutes (!) on Qt 6.5.1.

The change seems to be related to some underlying changes in QDateTime class implementation - specifically performance of copy constructors seems to have degraded a lot in Qt6.

I was able to solve the problem by switching implementation of datetimeFromNumber() to new QDateTime::fromMSecsSinceEpoch() function introduced in Qt6.5.

[dantti]

You are likely using a debug relase of Qt which indeed see degraded perfomance, still I don't like the ifdefs as this could be used byt Qt5 and 6, also it seems more interesting to have a QDateTime static than qint64 especially since we should be using std::chrono where possible

[alexagr]

This happens with Qt 6.5.1 in release (not debug) build using MSVC 2019 compiler.

[archibalduk]

This might be related to the issue and suggested fix I posted here: #322