[Qt] ensure all class attributes are init to 0

- in BitcoinGUI and UnitDisplayStatusBarControl
[Qt] use BitcoinGUI::DEFAULT_WALLET constant in bitcoin.cpp
[Qt] disable tray interactions when client model set to 0

- this prevents the ability to fiddle around with the system tray when
  already shutting down (e.g. on slow shutdowns because of a proxy delay)
- extends solution for #4360
Add boolean HaveWatchonly and signal NotifyWatchonlyChanged
[Qt] Add column Watch-only to transactions list
[Wallet] Improve ReorderTransactions(..)
ui_interface: remove unused NotifyBlocksChanged signal
Create new signal for notification of new blocks.  Use w/ -blocknotify
depends: add a fallback path in case package sources go missing

If a source url fails to download, try again at
$FALLBACK_DOWNLOAD_PATH/file.name, where FALLBACK_DOWNLOAD_PATH can be
overridden by the user.
Move UpdateTime to pow
Move CBlockIndex::GetBlockWork() to pow::GetProofIncrement(nBits)
replace ComputeMinWork with CheckMinWork
build: osx: Fix incomplete framework packaging for codesigning

Starting with 10.9, Framework versions must be signed individually, rather
than as a single bundle version, in order to be properly codesigned. This
change ensures that the proper plist files and symlinks are present prior to
packaging.
contrib/linearize: Guarantee that output is generated in-order

This was typically ensured implicitly by virtue of normal bitcoind
operation.  Adding an explicit check provides a stronger guarantee, and
it is cheap to add.
contrib/linearize: split block files based on year-month, not just year
Test IsPushOnly() with invalid push
Bypass reloading blocks from disk
Mark LevelDB "Read" and "Exists" functions as const.

Mark the "Read" and "Exists" functions in CLevelDBWrapper as "const".
They do not change anything in the DB, by definition.
Make appropriate getter-routines "const" in CCoinsView.

Mark the "Get"/"Have" routines in CCoinsView and subclasses as "const".
Use const CCoinsView's at some places.

At some places where it is possible (e. g., CheckInputs), use a const
version of CCoinsView instead of a non-const one.