[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
build: work around ccache/autotools warning-spamming bug

When using clang and ccache, builds spew lots of:
Clang: warning: argument unused during compilation

Upstream bug: https://bugzilla.samba.org/show_bug.cgi?id=8118

This is harmless, bug annoying. If ccache is being used and the
-Qunused-arguments flag is supported (clang), use it.
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
Test IsPushOnly() with invalid push
travis: initial descriptor
travis: add doc
Bypass reloading blocks from disk
Merge pull request #4749

425c7a8 travis: add doc (Cory Fields)
9380d01 travis: initial descriptor (Cory Fields)
386efb7 build: work around ccache/autotools warning-spamming bug (Cory Fields)
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.
Move CMedianFilter to timedata.cpp

Now that we no longer use the median filter to keep track of
the number of blocks of peers, that's the only place it is used.
Remove unused `alignup` function from util.h