qt: Show weeks as well as years behind for long timespans

[laanwj]

Four years looks slightly nicer than 208 weeks.

Closes #3811.

[felipelalli]

ship it!

[laanwj]

Have you tested?

[felipelalli]

No, I am sorry. Just code review. I am setting up the environment to start to contribute. I am reading all READMEs, and I read that I can contribute with code review and also test, which is the current bottleneck. Should I test carefully every change before send a "ship it"? Even theses small changes? Thank you for help.

[laanwj]

Nah, it's not super important in this case. I ask just in case, I usually feel better merging something when someone else has at least tested it (even minor changes like this). Thanks for trying to contribute!

[felipelalli]

Thank you @laanwj ! I hope to get everything over time.

[haakonn]

ACK.

I tested this using 'bitcoin-qt --reindex' under Qt4/Ubuntu. Seems to work as advertised, and I really like it! The only change that arguably could be an improvement would be to simplify "3 years and 0 weeks behind" to simply "3 years behind". But that's a minor thing and I think this is mergeable as is.