Adds Mac OS branch for the rotating log file opened at startup

[Lucassifoni]

Related to #353

I think we could remove the branching and standardize on getting a writable directory through QStandardPaths::writableLocation, what do you think ?

[gr5]

I prefer the way you did it where it only uses that function for mac. But @atsju could probably convince me otherwise. He wrote that code originally.

I approve this change but want @atsju to look at this before merging.

[atsju]

I agree with @gr5 I prefer it this way too. Single location is easyer to understand by end users even if it's old school.

On windows, the logs would end in C:/Users/<USER>/AppData/Roaming/<APPNAME>. A better place would probably be HomeLocation but I dislike when applications do that too.

[Lucassifoni]

I understand how the generic writable directory handed by the OS can be annoying for users who want to actually access the log. I'm fine with keeping the conditional then.

[atsju]

As this change does not affect any functionality and has no risk on windows I'm merging it.