Add timezone option to settings

[buccella]

In the current implementation, the "Transaction details" page shows the timestamp in the current system timezone. It would be useful to have a setting to specify a timezone, particularly for tax reporting purposes, to easily match transaction times against the historical market price of $OXEN.

[mdPlusPlus]

I don't see any times or dates in the wallet. But in case you're talking about the CSV export function, then it currently uses UTC and clearly states so.
However, personally I'd like to see it rather using the system time zone or simply the unixtimestamp.