Rebrand to `Bitcoin Core`

Only messages for now, executable names and other file names
can be changed later if necessary and safe.

Do not do an all-sweeping change. Some occurences of Bitcoin-Qt need to
be kept:

- Applicationname: this is used to determine the registry entry names,
  we don't want to lose settings over a silly name change.
- Where it refers to the executable name instead of the product name.
Merge pull request #3408

22f0135 Rebrand to `Bitcoin Core` (Wladimir J. van der Laan)
qt: Update English language file for translators
Merge pull request #3425

88ba264 qt: Update English language file for translators (Wladimir J. van der Laan)
Fix typo in unit-tests.md
Merge pull request #3430 from ldenman/patch-1

Fix typo in unit-tests.md