[CMAKE] Add a translate target

Summary:
This behaves like the autotools translate target, updating the
bitcoin_en.ts and bitcoinstrings.cpp files.

Test Plan:

ninja translate

Compare the output with autotools make translate: there are 2
additional translation strings from qt/splashscreen.cpp (after D5795).

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5796