[Qt] fix RecentRequestsTableModel function ambiuguity

- fixes a compiler ambiguity error with ::createIndex() called in
  RecentRequestsTableModel::index()
- also add some Q_UNUSED() macros
Squashed 'src/leveldb/' changes from 936b461..e991315

e991315 Merge upstream LevelDB 1.15.
0cfb990 Release LevelDB 1.15
02ac9f1 Merge upstream LevelDB 1.14.
0b9a89f Release LevelDB 1.14

git-subtree-dir: src/leveldb
git-subtree-split: e991315d7fe4ca84a98902578106cbffa3dcccfd
Merge src/leveldb changes for LevelDB 1.15
Remove unused ThreadSafeHandleURI from ui_interface
Remove unused ThreadSafeAskFee from ui_interface

ThreadSafeAskFee is effectively unused. It is only called
when the fAskFee parameter on SendMoney or SendMoneyToDestination
is true, which never happens. Remove it.
qt: status WalletModel::Aborted is no longer used

Aborting transactions happens in the GUI now as it should,
not the backend.
Seperate out wallet options in help message

Seperate out the wallet options in HelpMessage, and
don't show them if compiled with --disable-wallet.

Also add documentation for `-disablewallet` option.
[Qt] style-police, add missing license headers

- add missing license headers in Mac files
- small code formating cleanups
Rebrand README.md

How could I forgot this file...
Merge pull request #3405

55c6890 Squashed 'src/leveldb/' changes from 936b461..e991315 (Pieter Wuille)
Merge pull request #3435

9c6546c Rebrand README.md (Wladimir J. van der Laan)
Fix heading in README.md
Merge pull request #3381

7df07b3 [Qt] fix RecentRequestsTableModel function ambiuguity (Philip Kaufmann)
Merge pull request #3424

06eb2f2 Seperate out wallet options in help message (Wladimir J. van der Laan)
Merge pull request #3415

4a61c39 qt: status WalletModel::Aborted is no longer used (Wladimir J. van der Laan)
ca2c83d Remove unused ThreadSafeAskFee from ui_interface (Wladimir J. van der Laan)
37e67d3 Remove unused ThreadSafeHandleURI from ui_interface (Wladimir J. van der Laan)
Merge pull request #3433

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