fix(cpn): log viewer performance

[elecpower]

Fixes #7159

Summary of changes:

• replace QDateTime::fromString with custom conversion of dates and times
• perform date and time conversion once as log file is parsed
• add new hidden column to the log grid to hold the converted date and time and reference this column for all date and time usage
• add debug timers for possible future diagnostic use (though commented out)

[Johanl1964]

Have now tested the companion (Linux), now works really fast with large files, nice work!

[mha1]

Great job Neil. Tested with a 13MB log file (36848 lines). It's lightning fast.