Update Turkish translation

[KerimDemirkaynak]

I fixed some minor typos, added missing strings, and resolved issue #314.

[arch1t3cht]

I'm not a fan of manually adding strings to tr.po, this will make it harder to maintain in the future. In particular, the string for the help button mentioned in #314 should not be added to Aegisub's catalog - this string is used inside wxWidgets, not in Aegisub itself, and wxWidgets could update its usage at any time and break the translation (like it already has once).
If strings are missing, they should be accounted for in make_pot.sh so that they're automatically added to the pot file, and hence to the po files. If you see missing strings you can make issues for them (but the #314 one may take a while to complete).

I can merge the updates to the existing strings if you isolate those changes.

[KerimDemirkaynak]

Done.

[arch1t3cht]

Can you squash the commits to a single one and force push (or are you fine with me doing that)?

[KerimDemirkaynak]

I'd appreciate it if you could do it, I don't know much about GitHub.