Use base64 encoding also for datetime data - convert from QDateTime to qint64...

Description

Use base64 encoding also for datetime data - convert from QDateTime to qint64 and encode it prior to saving into XML. This is significantly faster than parsing QDateTime objects row by row in the XML.

Comparison for the test file attached in https://bugs.kde.org/show_bug.cgi?id=478508 and measured on i7-14700:

old format:

• size: 623kB
• load: 30s
• save: 5s

new format:

• size: 36.8kB
• load: 5s
• save: 0.5s

Conformity

• Changelog entry
• Unit tests
• Update INSTALL
• Downport
• Fixes: https://bugs.kde.org/show_bug.cgi?id=478508