Add portuguese translation

[BizarrePenguin]

I recently stumbled upon issue #155 and noticed that I wanted to do the portuguese translation some time ago. Here it is.

FYI: I am not a native speaker but have experience in portuguese since I lived in Brazil. I used a mixture of translation tools as well as my own experience for the translation.

[domai-tb]

Hey, thanks for your work! I've assigned this PR to @Noirox1 because he is tasked with the translation feature. As mentioned in #155, the current translations are not being used at all. So, I'm wondering if you could contribute to the portuguese translation again once we make all static strings translatable? I assume there will be more strings than those you've already translated. :)

[henry-herrmann]

@BizarrePenguin If you wish you can now continue with the translation using the .arb files in assets/l10n. Make sure to execute flutter gen-l10n once you're finished.

[BizarrePenguin]

Hey, thanks for notifying me. However I am a little lost now as I did not follow the development of the translation feature. Could you explain or point me in the right direction how I can re-do the translations and what needs to be generated? Also, which branch should I check out? I assume it won't work if I try to continue on my forked translation branch since it misses the new development of the translation feature, right?

Maybe it would be wise to update the Wiki too if the process of translating has changed.