[Qt] style-police, add missing license headers

- add missing license headers in Mac files
- small code formating cleanups
Merge pull request #3433

6c1bf19 [Qt] style-police, add missing license headers (Philip Kaufmann)
Remove ui_interface ThreadSafeAskFee residue

Missed these in ca2c83d (#3415).
Use mutex pointer instead of name for AssertLockHeld

This makes it useable for non-global locks such as the wallet and
keystore locks.
Document cs_wallet lock and add AssertLockHeld

Add locking assertions to wallet to all methods that
access internal fields and do not aquire the cs_wallet mutex.